An image forming apparatus is configured so that a first detection position and a second detection position are set along the direction of conveying a transfer material between adjacent conveyance rollers. The image forming apparatus has four sensors, two of which are disposed in the first detection position at a predetermined interval, and the remaining two of which are disposed in a second detection position at a predetermined interval. An actual length of the transfer material can be detected with high accuracy by performing (1) cancellation of an error in detection caused by variation in a speed at which the transfer material is conveyed, (2) cancellation of an error in detection caused by a skew, and (3) cancellation of an error in detection caused by oblique passing.