A server device configured to communicate with a door control device configured to control locking and unlocking a door installed at an entrance of a multifamily residential, the server device comprising: storage means configured to store, for each user, a service number set by a service provider; request reception means configured to receive, from the door control device, an unlock request including a device ID of the door control device and the service number; authentication means configured to execute an authentication based on information included in the unlock request received by the request reception means and information stored in the storage means; and unlock instruction means configured to transmit an unlock instruction to the door control device, when the authentication is succeeded; characterized in that the server device further comprising: notification means configured to notify the door control device when the authentication is failed, wherein: when there is a notification from the notification means, the request reception means is configured to receive, from the door control device, a reunlock request including identification information of the user as a resident of the multifamily residential, the identification information being different from the service number; and the authentication means is configured to reexecute the authentication based on the information included in the reunlock request received by the request reception means and the information stored in the storage means. request reception means configured to receive, from the door control device, an unlock request including a device ID of the door control device and the service number; authentication means configured to execute an authentication based on information included in the unlock request received by the request reception means and information stored in the storage means; and unlock instruction means configured to transmit an unlock instruction to the door control device, when the authentication is succeeded; characterized in that the server device further comprising: notification means configured to notify the door control device when the authentication is failed, wherein: when there is a notification from the notification means, the request reception means is configured to receive, from the door control device, a reunlock request including identification information of the user as a resident of the multifamily residential, the identification information being different from the service number; and the authentication means is configured to reexecute the authentication based on the information included in the reunlock request received by the request reception means and the information stored in the storage means. notification means configured to notify the door control device when the authentication is failed, wherein: when there is a notification from the notification means, the request reception means is configured to receive, from the door control device, a reunlock request including identification information of the user as a resident of the multifamily residential, the identification information being different from the service number; and the authentication means is configured to reexecute the authentication based on the information included in the reunlock request received by the request reception means and the information stored in the storage means.