An image forming apparatus including a developer regulating member and sealing members. The developer regulating member regulates an amount of the developer carried by a developer carrying member, and has a center region having a first outer diameter of a first longitudinal cross-sectional region which increases toward an axial center portion of the developer regulating member according to a predetermined calculated amount of deflection of the developer regulating member, side regions located at axial end portions and having a second outer diameter of a second longitudinal cross-sectional region greater than a third outer diameter of a third longitudinal cross-sectional region obtained according to deflection of the developer regulating member. The sealing members have respective inner edges which sandwich boundaries of the center region and each of the side regions.