A communication system includes a registration server, a management server and a first target device. When receiving destination information from a communication device, the registration server transmits authentication information to the communication device, transmits screen relating information to a destination indicated by the destination information, receives user information from the communication device and registers the user information therein. When receiving the authentication information from the communication device, the first target device transmits the authentication information and first device identification information to the management server. When receiving the authentication information and the first device identification information from the first target device, the management server registers the first device identification information therein. The user information in the registration server and the first device identification information in the management server are registered with being associated with each other.