To provide a technique for improving suppression effect of a harmonic current in heating control of a heater.

A heating device comprises: a switching part for switching on/off of electrical-conduction from an AC power supply to a heating body; a temperature detecting part for detecting a temperature of the heating body; and a conduction control part for performing a first conduction mode of changing the power supply ratio (DUTY ratio) with respect to a unit time from an AC power supply to a heating body by controlling the switching of the switching part so that the detection temperature of the temperature detecting part is within a target range. During performing the first conduction mod (time t0 to t6), the conduction control part performs a second conduction mode (time t2 to t3) of fixing the power supply ratio to approximately 100% or almost 0%.