Image forming system

An image forming system includes an image forming apparatus, a management terminal device, and a mobile terminal device. The mobile terminal device downloads and installs an application program, presents identification information to the management terminal device, and accepts a temporal ID. The management terminal device, upon accepting the identification information from the mobile terminal device, generates the temporal ID, presents the temporal ID to the mobile terminal device, and transmits the temporal ID to a user authentication device to register the temporal ID as a user ID of the guest user. When payment of a usage fee for the image forming apparatus by the guest user is completed, the management terminal device deletes registration of the temporal ID. The image forming apparatus performs user authentication when the guest user logs in to the image forming apparatus, and issues an invoice when the guest user logs out.