PURPOSE: To minimize the soiling of a belt with an adhering toner from an image carrier so as to reduce the load of a cleaning device and thus maintain the cleaning performance of the device over a long period of time by controlling the driving of a roller length variation means during transfer to correspond to the size of transfer paper to which images are to be transferred.

CONSTITUTION: When copying conditions are set, a controller calculates transfer width and then drives a reversible motor in either normal or reverse direction in accordance with the calculated transfer width, so that the length of a transfer roller 88 corresponds to the transfer width. While the reversible motor is being driven, a detection signal from a potentiometer 96 is utilized to check whether or not a predetermined value corresponding to the transfer width calculated has been reached, and with the detection signal from the potentiometer 96 reaching the predetermined value, the driving of the reversible motor is stopped. With the transfer roller 88 thus adjusted to the length matching the width of the transfer paper used, the controller turns a solenoid device 937 on to energize the solenoid device 937.