A developer cartridge and developing device are provided. The developer cartridge is attachable to and detachable from a developing housing, the developing housing comprising a drive gear for transmitting a driving force to the developer cartridge. The developer cartridge includes a cartridge housing that is configured to accommodate developer; a first passage portion provided in the cartridge housing to allow a passage of a detection light for detecting an amount of the developer accommodated in the cartridge housing; an agitator provided in the cartridge housing and configured to agitate the developer by the driving force; a transmission gear configured to mesh with the drive gear to transmit the driving force to the agitator, the transmission gear meshing with the drive gear when the developer cartridge is installed in the developing housing; and a second passage portion provided in the transmission gear to allow the passage of the detection light.