PURPOSE: To select an optimum mode according to an original state and to enhance efficiency also from a viewpoint of saving energy by providing a detection means detecting the number of sheets of set original and a mode setting means, etc., selecting an image forming mode.

CONSTITUTION: This device is provided with a one original sheet discrimination means 134 discriminating that the number of sheets of the original detected by number of original sheets detection means 132, 133 is one sheet and a mode setting means 135 selectively setting plural copy modes containing a one-side →one-side mode forming an image of one sheet of one-side original on one side of one sheet of transfer material and a one-side → both-side mode forming two sheets of the image of one-side original on both sides of one sheet of transfer material. Further, the device is constituted so as to contain a both-side mode switch means 136, etc., switching the copy mode from the one-side →both-side mode to the one-side → one-side mode and making the selection state of the one-side → one-side mode. Thus, the optimum mode is selected according to the original state of an original transfer means 60, and the efficiency is enhanced also on the viewpoint of saving energy.