A CPU causes a power control unit and a bias application circuit at all times to apply a developing bias to a developing roller thereby to execute a printing operation. When the CPU determines the lapse of lifetime of a developer cartridge on the basis of detection signals from a toner shortage detecting sensor and a counter, the CPU detects that the developer cartridge is at a spaced position, and causes the power control unit to feed a fusing current from the bias application circuit thereby to fuse a fuse element.