PURPOSE: To stably supply developer to a gap between an electrostatic latent image carrier and a developing sleeve by providing a mapping height regulation member at a part where the composite sum of magnetic force in a tangent line direction and magnetic force in a normal line direction on the developing sleeve is the largest.

CONSTITUTION: The developing sleeve 1A which is opposed and arranged on the electrostatic latent image carrier 4, a magnetic roller which is provided with N poles and S poles on the outside circumference part and incorporated in the sleeve 1A and the mapping height regulation member 3 which is opposed to the outside circumferential surface of the sleeve 1A by keeping prescribed interval are provided. Then, the developer held on the outside circumferential surface of the sleeve 1A is carried in one direction by the magnetic force of the roller 2 while the carrying quantity of the developer is regulated by the regulation member 3 with the aid of using the rotation of the sleeve 1A. The regulation member 3 is provided at the part of near the part where the composite sum (3) of the magnetic force in the tangent line direction and in the normal line direction on the sleeve 1A is the largest between two adjacent magnetic poles of the roller 2. Thus, the developer is stably supplied to the gap between the electrostatic latent image carrier 4 and the developing sleeve 1A.