A heater includes: a planar heat generator; a power supply circuit that controls supply of power to the planar heat generator; a plurality of temperature sensors that is provided on the planar heat generator and measures a temperature; and a hardware processor that detects abnormality of the temperature sensor in a case where a difference in temperature measured by each of two of the temperature sensors out of the plurality of temperature sensors exceeds an abnormality threshold value after a predetermined waiting time has elapsed since the supply of power to the planar heat generator is stopped.