PURPOSE: To detect the main pole position of a developing magnetic roller without requiring labor, by utilizing a magnetism-sensitive element and a microcomputer provided so as to be revolved around the developing magnetic roller.

CONSTITUTION: A magnetism-sensitive element 3 is sent by an actuator 5, from its original position in the sense of circumferential direction, as set forth in time, of a magnetic roller 1 by a prescribed distance at every one pitch. At this time, the output of a gauss meter 4 is inputted to a microcomputer 7 at every one step feed and stored in RAM10. CPU8 of the microcomputer 7 calculates the main pole position of the magnetic roller 7 from the data stored in RAM10 and the calculated value is compared with the proper position of the main pole of the magnetic roller 1 preliminarily stored in ROM9 and discrim inated. When the difference thereof exceeds an allowable value, an alarm display device 14 is operated to issue an alarm.