A heater includes a plurality of resistive heat generators electrically connected to each other in parallel. A power supply supplies power to the resistive heat generators. An electric current detector detects an electric current that flows through the resistive heat generators. A voltage detector detects a voltage applied to the resistive heat generators. An electric current controller controls the electric current that flows through the resistive heat generators based on the electric current detected by the electric current detector and the voltage detected by the voltage detector. The electric current detector detects the electric current in a state in which, after the power supply starts supplying the power to the resistive heat generators, a waveform of an alternating current supplied to the resistive heat generators remains constant for a predetermined time period or longer taken for the electric current detector to detect the electric current.