PURPOSE: To stably secure constant image density, by measuring a charge amount of a toner as a surface potential of a toner layer, controlling movement of a blade in accordance with the measured value, and varying thickness of the toner layer.

CONSTITUTION: The surface potential of a toner layer 9 applied on the surface of a sleeve 2 is detected by a sensor 10, and in accordance with its detecting signal, the surface potential measured by a potential measuring circuit 11 is supplied to a controlling circuit 13, a blade moving mechanism 12 is controlled, and thickness of the toner layer is varied by moving a blade 7 far and near to a sleeve 2 so that the surface potential of the toner layer 9 becomes equal to the reference potential set to the potential measuring circuit 11. In this way, a constant developing amount, that is to say, constant image density can be secured stably, in spite of a variation of a charge amount of a toner.