PURPOSE: To obtain an inexpensive and lightweight process unit by installing a detecting means for toner ratio concentration in a developer in the main body of the image forming device which has the detachable electrophotographic process unit.

CONSTITUTION: The detecting means 164 for the toner ratio density in the developer is provided on the side of the image forming device main body 1. A sticking medium for toner is installed as the toner ratio concentration detecting means 164 on the side of, for example, the process unit 3 and an optical sensor is provided on the side of the image forming device main body 1 to detect the toner ratio concentration by utilizing the inverse proportion between of the amount of toner sticking on the sticking medium and the reflection factor of the toner sticking medium. Consequently, even when the process unit is replaced, the toner ratio concentration detecting means can be used repeatedly, so the inexpensive and lightweight process unit can be obtained.