An image forming system is described which is capable of assembling an output product which consists of sheets having uniform sheet widths by appropriately performing a trimming process even if the output product is made from a plurality of paper types. The image forming system consists of an image forming apparatus configured to form an image on a sheet; a finisher provided with a slit unit for trimming edges of sheets and capable of assembling an output product by joining the trimmed sheets, and a control unit for controlling the image forming apparatus and the finisher. When a book cover (standard paper type sheet) is supplied, the control unit trims the paper edges by a designated trimming width. Conversely, when a book body (non-standard paper type sheet) is supplied, the control unit trims the paper edges by a designated trimming width to which an offset amount is added.