First fixed value is set as decreasing variable number C and second fixed value corresponding to measured resistance value of a thermal head is determined as constant K(R). Energization of the thermal head is started immediately after start of printing period and the energization is kept until lapse of pulse application time calculated based on chopping duty ratio determined based on thermal head voltage and the constant K(R). Engergization of the thermal head is stopped at lapse of the pulse application time and withheld until lapse of unit time. Calculation value (C(V)×K(TH)) calculated based on temperature and voltage of the thermal head is decremented from the current decreasing variable number C to obtain renewed decreasing variable number C. When the renewed decreasing variable number C is larger than 0, serial processes of calculating pulse application time, keeping and withholding energization of the thermal head are repeated.