To allow an electronic signature system to be inexpensively introduced by eliminating the need for managing a user's secret key in an image forming apparatus.

An image forming system includes: one or a plurality of image forming apparatus 1 connected to a network 2 so as to add an electronic signature to document data; and a signature server device 4 connected to the network 2 so as to provide an electronic signature of a log-in user of the image forming apparatus 1 about document data to the image forming apparatus 1, in response to an electronic signature request from the image forming apparatus 1.