The fixing apparatus includes a heater including at least two heat generation members, a relay, a triac, a zero-crossing circuit unit connected between a first pole and a second pole of an AC power supply, and configured to output a zero-crossing signal, and a CPU configured to control the relay and the triac, and the CPU determines which one of the at least two heat generation members is the heat generation member to which electric power is being supplied from the AC power supply, based on the zero-crossing signal output from the zero-crossing circuit unit.