A drum unit includes a photosensitive drum, a developing unit, a toner cartridge, a connector and a relay board. The photosensitive drum is rotatable about a first axis extending in an axial direction. The developing unit includes a magnetic roller, a developer container, a conveyer member and a toner sensor. The developer container accommodates therein carrier, and has an inlet. The conveyer member is positioned in the developer container, and configured to convey toner and the carrier from the inlet toward the magnetic roller. The toner cartridge accommodates therein toner and is attachable to the developer container to replenish toner in the developer container through the inlet. The connector is electrically connected to an image forming apparatus. The relay board electrically connects the toner sensor to the connector. The relay board is configured to transmit a detection signal from the toner sensor to the image forming apparatus.