PURPOSE: To prevent the occurrence of a defective image by providing between the drive roller and the follower roller, both of which support a developer carrier belt, an elastic body roller, which is larger than both the rollers in diameter, thereby stabilizing the running conditions of the developing belt.

CONSTITUTION: The roller 10, made of an elastic body (for example, sponge) is arranged between the drive roller 3 and the follower roller 4. By making, in this constitution, the diameter of the elastic body roller 10 larger than that of the drive roller 3 and that of the follower roller 4, the running of the developing belt 2 stabilizes without increasing frictional forces between the developing belt 2 and the photosensitive body 1 and the developing roller 6. Further, the contact-widths of the photosensitive body 1 and the developing roller 6 with respect to the developing belt 2 can be controlled by the adjustments of the size and relative positions of the drive roller 3, follower roller 4 and elastic body roller 10, so that a stable image can be obtained at all times.