PURPOSE: To obtain picture printing with a constant density and stable quality by a method in which signals corresponding to the ambient temperatures of a thermal head are fed-back to a simulation circuit for controlling temperatures.

CONSTITUTION: The ambient temperatures of a thermal head are detected by a temperature sensor Si-D, converted into voltage by an operating amplifier OP, and supplied to a simulator circuit (Rs, Cs). In case where the ambient temperature of the thermal head is high, low voltage is supplied to an inverter 22, and when the ambient temperature is low, a high voltage is supplied to the inverter 22. The output of the inverter 22 is added to an instable applied voltage Vin and given as a control voltage to VCO20. Accordingly, the output signal of the VCO20 changes into high-frequency signal in proportion to input voltage, and thereby the output time of carry signal of a counter 21 is hastened and the time (time taken for SW1 to become off) taken for FF9 to become reset is hastened. Pulse from an output terminal T2 is supplied to the thermal head, while the heat amount applied to the head becomes constant equivalently, and the quality of picture is stabilized.