An image forming apparatus includes an alternating-current high-voltage power supply generating an alternating-current bias for detection of the remaining amount of toner. The alternating-current high-voltage power supply includes a piezoelectric transformer, a piezoelectric-transformer driving unit, a piezoelectric-transformer driving-signal generating unit, a voltage detecting unit, a voltage setting unit, and an controlling unit that feeds back a difference signal between a detection level signal supplied from a detection signal detecting unit and a setting signal supplied from the voltage setting unit to the piezoelectric-transformer driving-signal generating unit to control an output voltage from the piezoelectric-transformer.