A developer cartridge includes a casing, a development roller, a support member, and at least one transmission gear. The development roller is rotatably disposed in the casing. The support member is provided as a separate member from the casing and is mounted on the casing. The at least one transmission gear is rotatably supported by the support member. The at least one transmission gear transmits a driving force to the development roller. The support member is configured to be dismounted from the casing together with the at least one transmission gear.