diff --git a/Mastodon/Scene/Onboarding/PickServer/MastodonPickServerViewController.swift b/Mastodon/Scene/Onboarding/PickServer/MastodonPickServerViewController.swift
index 68570971..638734c1 100644
--- a/Mastodon/Scene/Onboarding/PickServer/MastodonPickServerViewController.swift
+++ b/Mastodon/Scene/Onboarding/PickServer/MastodonPickServerViewController.swift
@@ -336,6 +336,7 @@ extension MastodonPickServerViewController {
                 } else {
                     let mastodonRegisterViewModel = MastodonRegisterViewModel(
                         domain: server.domain,
+                        context: self.context,
                         authenticateInfo: response.authenticateInfo,
                         instance: response.instance.value,
                         applicationToken: response.applicationToken.value
diff --git a/Mastodon/Scene/Onboarding/Register/MastodonRegisterViewModel.swift b/Mastodon/Scene/Onboarding/Register/MastodonRegisterViewModel.swift
index cd6106c2..919443f2 100644
--- a/Mastodon/Scene/Onboarding/Register/MastodonRegisterViewModel.swift
+++ b/Mastodon/Scene/Onboarding/Register/MastodonRegisterViewModel.swift
@@ -18,6 +18,7 @@ final class MastodonRegisterViewModel {
     let authenticateInfo: AuthenticationViewModel.AuthenticateInfo
     let instance: Mastodon.Entity.Instance
     let applicationToken: Mastodon.Entity.Token
+    let context: AppContext
     
     let username = CurrentValueSubject<String, Never>("")
     let displayName = CurrentValueSubject<String, Never>("")
@@ -46,11 +47,13 @@ final class MastodonRegisterViewModel {
 
     init(
         domain: String,
+        context: AppContext,
         authenticateInfo: AuthenticationViewModel.AuthenticateInfo,
         instance: Mastodon.Entity.Instance,
         applicationToken: Mastodon.Entity.Token
     ) {
         self.domain = domain
+        self.context = context
         self.authenticateInfo = authenticateInfo
         self.instance = instance
         self.applicationToken = applicationToken
@@ -78,6 +81,21 @@ final class MastodonRegisterViewModel {
             }
             .assign(to: \.value, on: usernameValidateState)
             .store(in: &disposeBag)
+        
+        username.debounce(for: .milliseconds(300), scheduler: DispatchQueue.main).removeDuplicates()
+            .sink { [weak self] text in
+                self?.lookupAccount(by: text)
+            }
+            .store(in: &disposeBag)
+        
+        usernameValidateState
+            .sink { [weak self] validateState in
+                if validateState == .valid {
+                    self?.usernameErrorPrompt.value = nil
+                }
+            }
+            .store(in: &disposeBag)
+
         displayName
             .map { displayname in
                 guard !displayname.isEmpty else { return .empty }
@@ -145,6 +163,23 @@ final class MastodonRegisterViewModel {
         .assign(to: \.value, on: isAllValid)
         .store(in: &disposeBag)
     }
+    
+    func lookupAccount(by acct: String) {
+        if acct.isEmpty {
+            return
+        }
+        let query = Mastodon.API.Account.AccountLookupQuery(acct: acct)
+        context.apiService.accountLookup(domain: domain, query: query, authorization: applicationAuthorization)
+            .sink { _ in
+
+            } receiveValue: { [weak self] account in
+                guard let self = self else { return }
+                let text = L10n.Scene.Register.Error.Reason.taken(L10n.Scene.Register.Error.Item.username)
+                self.usernameErrorPrompt.value = MastodonRegisterViewModel.errorPromptAttributedString(for: text)
+            }
+            .store(in: &disposeBag)
+
+    }
 }
 
 extension MastodonRegisterViewModel {
diff --git a/Mastodon/Scene/Onboarding/ServerRules/MastodonServerRulesViewController.swift b/Mastodon/Scene/Onboarding/ServerRules/MastodonServerRulesViewController.swift
index fb86e81e..447896c2 100644
--- a/Mastodon/Scene/Onboarding/ServerRules/MastodonServerRulesViewController.swift
+++ b/Mastodon/Scene/Onboarding/ServerRules/MastodonServerRulesViewController.swift
@@ -204,7 +204,7 @@ extension MastodonServerRulesViewController {
     @objc private func confirmButtonPressed(_ sender: UIButton) {
         os_log(.info, log: .debug, "%{public}s[%{public}ld], %{public}s", ((#file as NSString).lastPathComponent), #line, #function)
 
-        let viewModel = MastodonRegisterViewModel(domain: self.viewModel.domain, authenticateInfo: self.viewModel.authenticateInfo, instance: self.viewModel.instance, applicationToken: self.viewModel.applicationToken)
+        let viewModel = MastodonRegisterViewModel(domain: self.viewModel.domain,context: self.context, authenticateInfo: self.viewModel.authenticateInfo, instance: self.viewModel.instance, applicationToken: self.viewModel.applicationToken)
         self.coordinator.present(scene: .mastodonRegister(viewModel: viewModel), from: self, transition: .show)
     }
 }
diff --git a/Mastodon/Service/APIService/APIService+Account.swift b/Mastodon/Service/APIService/APIService+Account.swift
index 04908514..7638f244 100644
--- a/Mastodon/Service/APIService/APIService+Account.swift
+++ b/Mastodon/Service/APIService/APIService+Account.swift
@@ -152,4 +152,17 @@ extension APIService {
         )
     }
     
+    func accountLookup(
+        domain: String,
+        query: Mastodon.API.Account.AccountLookupQuery,
+        authorization: Mastodon.API.OAuth.Authorization
+    ) -> AnyPublisher<Mastodon.Response.Content<Mastodon.Entity.Account>, Error> {
+        return Mastodon.API.Account.lookupAccount(
+            session: session,
+            domain: domain,
+            query: query,
+            authorization: authorization
+        )
+    }
+    
 }
diff --git a/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+Account.swift b/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+Account.swift
index 0f98dbe0..78f338b6 100644
--- a/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+Account.swift
+++ b/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+Account.swift
@@ -132,3 +132,54 @@ extension Mastodon.API.Account {
     }
     
 }
+
+extension Mastodon.API.Account {
+    static func accountsLookupEndpointURL(domain: String) -> URL {
+        return Mastodon.API.endpointURL(domain: domain).appendingPathComponent("accounts/lookup")
+    }
+
+    public struct AccountLookupQuery: GetQuery {
+
+        public var acct: String
+        
+        public init(acct: String) {
+            self.acct = acct
+        }
+        
+        var queryItems: [URLQueryItem]? {
+            var items: [URLQueryItem] = []
+            items.append(URLQueryItem(name: "acct", value: acct))
+            return items
+        }
+    }
+    
+    /// lookup account by acct.
+    ///
+    /// - Version: 3.3.1
+
+    /// - Parameters:
+    ///   - session: `URLSession`
+    ///   - domain: Mastodon instance domain. e.g. "example.com"
+    ///   - query: `AccountInfoQuery` with account query information,
+    ///   - authorization: user token
+    /// - Returns: `AnyPublisher` contains `Account` nested in the response
+    public static func lookupAccount(
+        session: URLSession,
+        domain: String,
+        query: AccountLookupQuery,
+        authorization: Mastodon.API.OAuth.Authorization?
+    ) -> AnyPublisher<Mastodon.Response.Content<Mastodon.Entity.Account>, Error> {
+        let request = Mastodon.API.get(
+            url: accountsLookupEndpointURL(domain: domain),
+            query: query,
+            authorization: authorization
+        )
+        return session.dataTaskPublisher(for: request)
+            .tryMap { data, response in
+                let value = try Mastodon.API.decode(type: Mastodon.Entity.Account.self, from: data, response: response)
+                return Mastodon.Response.Content(value: value, response: response)
+            }
+            .eraseToAnyPublisher()
+    }
+    
+}