To form a proper image and perform the positioning of a sheet tip part without increasing frictional force to a carrier guide part or the like.

Slack is formed at a sheet tip part by abutting positioning means 33a, 33b on a tip part of a sheet P which is selectively carried from a sheet carrier path and the positioning in a carrier width direction of the sheet tip part is performed. After slack is formed by abutting the sheet P on the positioning means 33a, 33b, a carrier means 32 (41) is stopped and slack formation amount of the sheet tip part at the time of the positioning is changed according to the shape of the sheet carrier path by changing time when the carrier means 32 (41) is stopped after the sheet P is abutted on the positioning means 33a, 33b.