HTTPステータスコード・チートシート

最もよく使うHTTPステータスコードをクラス別に素早く見渡せる要約です。

毎回スペックを調べなくても済むように、実務で最もよく出会うステータスコードをクラス別にまとめました。下のリストの各コードを押すと、意味・発生する状況・原因・解決方法を載せた詳細ページに移動します。

素早い判断基準: 2xxは成功、3xxは移動（リダイレクト）、4xxはリクエスト（クライアント）の問題、5xxはサーバーの問題です。4xxが見えたらリクエストのURL・ヘッダー・認証を、5xxが見えたらサーバーログとバックエンドの状態をまず確認してください。

よく使うステータスコードを一目で確認

2xx成功 (Success)
200OK よく使うThe request succeeded. The response body depends on the method used.201Created よく使うThe request succeeded and a new resource was created as a result.204No Content よく使うThe request succeeded but there is no content to return.
3xxリダイレクト (Redirection)
301Moved Permanently よく使うThe resource has permanently moved to a new URL given in the Location header.302Found よく使うThe resource temporarily resides at a different URL; keep using the original URL.304Not Modified よく使うThe cached copy is still valid, so no body is sent.
4xxクライアントエラー (Client Error)
400Bad Request よく使うThe server cannot process the request due to a client error such as malformed syntax.401Unauthorized よく使うAuthentication is required and has failed or not been provided (i.e. unauthenticated).403Forbidden よく使うThe server understood the request but refuses to authorize it. Re-authenticating will not help.404Not Found よく使うThe server cannot find the requested resource. The most well-known status code.405Method Not Allowed よく使うThe HTTP method is known but not supported for the target resource.409Conflict よく使うThe request conflicts with the current state of the resource.422Unprocessable Content よく使うThe request is well-formed but has semantic errors (e.g. validation failed).429Too Many Requests よく使うThe client has sent too many requests in a given time (rate limiting).
5xxサーバーエラー (Server Error)
500Internal Server Error よく使うA generic catch-all: the server hit an unexpected condition and could not complete the request.502Bad Gateway よく使うA gateway or proxy received an invalid response from an upstream server.503Service Unavailable よく使うThe server is temporarily unable to handle the request due to overload or maintenance.504Gateway Timeout よく使うA gateway or proxy did not get a timely response from an upstream server.