PURPOSE: To make a little device small in size and light in weight, and also to make the service life of plural developers uniform by constituting said device so that the diameter of a rotating body is different from each other in accordance with the use frequency of plural developing devices.

CONSTITUTION: The outside diameter of a magnet roller 43 of the first developing device 14 for containing developers 42 of blue, red and yellow whose consumption quantity is small is made small, and the diameter of a magnet roller 46 of the second developing device 15 for containing a developer of black whose consumption quantity is large is made large. Accordingly, the device is made small in size and light in weight without deteriorating its copying performance, and the service life of the developers 42, 45 can be made uniform.