To provide a method of binding medium sheets which enables appropriate heating of binding regions through a large number of sheets or a stack.

The method of binding a plurality of medium sheets 10 has a step of adding an image forming material to the binding regions of single medium sheets 10 and activating the material, a step of gathering the sheets 10 into the stack 14 together with the sheets 10 gathered previously, a step of reactivating the image forming material added to the binding regions 12 of the sheets 10, and a step of repeating the above adding, gathering and reactivating operations for each of the sheets 10 in a plurality.