Development apparatus and cartridge with sealing member to prevent leakage of developer

A developing apparatus for an image forming apparatus includes a frame having a developer container; a developing roller; a blade for regulating a developer layer thickness on the roller, the blade including a base end portion supported by the frame, and a free end portion contacted to the roller; and an elastic seal provided between the frame and the blade to prevent leakage of the developer from the developer container in an axial direction of the roller at an end portion, wherein in a state before the roller is mounted to the frame, a thickness of the seal, measured in a compression direction in which the seal is compressed by the blade, is larger in a free end portion side than in a base end portion side with respect to a direction crossing with the axial direction.