PURPOSE: To secure the fixed or more quantity of developer which should be detected by a toner sensor and to effectively evade the defective detection of the toner sensor even in the case that the flowing of the developer scraped by a developer quantity regulation member directs to a part where the toner sensor is installed and the quantity of the developer supplied to a developing roller is small because the flowability of the developer is deteriorated.

CONSTITUTION: This device is the developing device for two-component system developer which is provided with the developing roller 3 for supplying the developer to a developing area, the developer quantity regulation member 10 which is provided at a fixed interval with respect to the surface of the roller 3 and which substantially extends over the whole of the shaft direction of the roller 3 and the toner sensor 15 for detecting the concentration of toner in the developer at the central part of the width direction of a supporting body 12 for holding the regulation member 10. Besides, a wall surface 12a being an upstream side with respect to the flowing of the developer of the supporting bode 12 is made a continuous surface whose form is recessed toward the central part where the sensor 15 is provided.