A heating member applies heat to a toner image formed on a sheet and fixes the toner image to the sheet. A bidirectional thyristor controls whether to supply power from an alternating current (AC) power source to the heating member. A direct current (DC) voltage source supplies a control signal to a gate terminal of the bidirectional thyristor. An obtaining circuit obtains a state parameter including at least one among a temperature of the bidirectional thyristor, an AC voltage supplied from the AC power source, and an operational history of the DC voltage source. A controller controls the number of outputs of the control signal outputted from the DC voltage source to the gate terminal in a half cycle of the AC voltage according to the state parameter.