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 controls the power supply circuit in a predetermined condition to maintain a temperature measured by at least one of the plurality of temperature sensors at a target value, wherein the planar heat generator includes: an insulator; and a heat generator disposed on the insulator, the heat generator includes each of a plurality of linear parts extending in parallel with each other at a predetermined interval P, and each of the plurality of temperature sensors is disposed along a direction orthogonal to an extending direction of each of the plurality of linear parts at a position mutually shifted by a predetermined distance D.