A developer cartridge includes a developer cartridge case for accommodating a developer, a developing roller, a toner layer thickness control blade, and a side sealing member. The developing roller is supported by the developer cartridge case such that a circumferential surface thereof is exposed to an outside of the developer cartridge case along a width direction. The toner layer thickness control blade includes a blade body portion made of a metallic plate. A tip portion of the blade body portion is disposed at a sliding portion of the developing roller and the side sealing member so as to contact with the side sealing member. A frictional heat generated at the sliding portion is released to the outside of the developer cartridge case through the blade body portion in order restrain a leak of the developer as much as possible.