PURPOSE: To reduce the change in printing density due to the change in the temperature of a thermal head, by a method wherein the temperature of a head of a thermal printer is detected by a temperature detector, and the energy supplied to the head is controlled according to the detection result.

CONSTITUTION: The thermal head 4 is constructed by fitting a heating element 9 on a head substrate 8 which is formed of a ceramic or the like on a metallic support 7 for the head. For example, the temperature detector 5 is fitted to the center of the metallic support 7 to detect the temperature of the head 4. The temperature of the thermal head 4 is detected by the temperature detector 5, and the voltage of a DC power source E supplied to the heating element 9 of the head 4 is controlled by a voltage-controlling circuit 6. By performing this control, the change in the printing density is eliminated.