An image forming apparatus capable of reducing a part of a sheet exposed outside the apparatus on standby after finishing printing a first side in both-sided printing. A transfer material P having an image formed on its first side is fed until a part thereof gets exposed outside a full-color printer 1 and is fed to a refeeding path in the full-color printer 1. The transfer material P inverted by a flapper 44 is fed to a standby position to form an image on a second side thereof and is stopped there. In the case where a preparation for forming the image on the second side is not completed, the standby position for having the transfer material P stopped is changed corresponding to length of the inverted transfer material P in a feeding direction.