PURPOSE: To correct the deterioration of a density gradation caused by a change in thermal head temperature with high accuracy.

CONSTITUTION: Strobe data in accordance with the temperature of a thermal head 3 is stored in a look-up table 21. The temperature of the thermal head 3 is detected, and strobe data based on this temperature is transmitted from the look-up table 21 to a strobe controller 2. Based on the strobe data, the strobe controller 22 sets a strobe time. Namely, with a high thermal head temperature, a short strobe time is set, and a low thermal head temperature, a long strobe time is set. In this manner, independently of the temperature of the thermal head, a good gradation can be expressed. Furthermore, by storing line preheat data in the look-up table 21, the thermal head is preheated at the start of printing to ensure a gradation expression from the first line.