According to an embodiment, an image forming apparatus includes an image bearing body, a developing unit, a supplying unit, a detecting unit, and a control unit. The supplying unit supplies a recording agent from a recording agent cartridge to the developing unit. The detecting unit detects a residual amount of the recording agent. The control unit controls, if the recording agent cartridge is not replaced, the supplying unit to supply the recording agent to the developing unit if the detected residual amount decreases to a first residual amount or less and controls, if the recording agent cartridge is replaced, the supplying unit to supply the recording agent in the recording agent cartridge after the replacement to the developing unit if the detected residual amount of the recording agent in the recording agent cartridge before the replacement decreases to a second residual amount or less.