An authority transfer system including a client, user agent, a resource server, and an authorization server for authorizing the client to access the resource server, the authority transfer system, comprising: a processor and a memory coupled to the processor and having stored thereon instructions, which when executed by the processor, and cooperating to act as: transmitting means configured to transmit (S4.0), from the client to the authorization server via the user agent, an authorization code request for authorization by the authorization server for access to the resource server by the client; receiving means configured to receive an authorization code response, the authorization code response being a response to the authorization code request, characterized in that: the authorization code request includes response destination information for designating a response destination to which the authorization server is to return the authorization code response and signature information added to the response destination information; the authority transfer system further comprises responding means configured to return (S1.7), by the authorization server, the authorization code response to the authorization code request based on the response destination information included in the authorization code request transmitted by the transmitting means; and wherein the authorization server is configured to verify the signature information added to the response destination information by using a private key held by the authorization server. a processor and a memory coupled to the processor and having stored thereon instructions, which when executed by the processor, and cooperating to act as: transmitting means configured to transmit (S4.0), from the client to the authorization server via the user agent, an authorization code request for authorization by the authorization server for access to the resource server by the client; receiving means configured to receive an authorization code response, the authorization code response being a response to the authorization code request, characterized in that: the authorization code request includes response destination information for designating a response destination to which the authorization server is to return the authorization code response and signature information added to the response destination information; the authority transfer system further comprises responding means configured to return (S1.7), by the authorization server, the authorization code response to the authorization code request based on the response destination information included in the authorization code request transmitted by the transmitting means; and wherein the authorization server is configured to verify the signature information added to the response destination information by using a private key held by the authorization server. transmitting means configured to transmit (S4.0), from the client to the authorization server via the user agent, an authorization code request for authorization by the authorization server for access to the resource server by the client; receiving means configured to receive an authorization code response, the authorization code response being a response to the authorization code request, characterized in that: the authorization code request includes response destination information for designating a response destination to which the authorization server is to return the authorization code response and signature information added to the response destination information; the authority transfer system further comprises responding means configured to return (S1.7), by the authorization server, the authorization code response to the authorization code request based on the response destination information included in the authorization code request transmitted by the transmitting means; and wherein the authorization server is configured to verify the signature information added to the response destination information by using a private key held by the authorization server. the authorization code request includes response destination information for designating a response destination to which the authorization server is to return the authorization code response and signature information added to the response destination information; the authority transfer system further comprises responding means configured to return (S1.7), by the authorization server, the authorization code response to the authorization code request based on the response destination information included in the authorization code request transmitted by the transmitting means; and wherein the authorization server is configured to verify the signature information added to the response destination information by using a private key held by the authorization server.