To eliminate a problem with image forming caused by meandering of a belt with simple constitution by calculating a belt meandering quantity according to edge position information, and correcting an image forming position in the belt width direction.

A belt meandering quantity in positions of image forming units K, Y, M and C is calculated by a meandering quantity calculating circuit 10 on the basis of the relative positional relationship between a belt position regulating edge guide 6, an edge detecting sensor 7 to detect the edge and the image forming units K, Y, M and C and edge position information obtained from the edge detecting sensor 7. An image forming position in the belt width direction in the image forming units K, Y, M and C is corrected by correcting means 11 and 15 on the basis of an obtained belt meandering quantity. Since the image forming position in the belt width direction in the image forming units is properly corrected according to the belt meandering quantity, inconvenience at image forming time caused by meandering of a belt can be eliminated without using the conventional complicated and large-scale device constitution.