PURPOSE: To make the substantial reduction of the driving power source capacity of thermal head, by a method wherein printing is carried out by dividing a thermal head into plural segments when the amount of data exceeds a setting value, and the driving voltage of thermal head is detected. After said driving voltage after printing returns to the setting value, the next printing is carried out.

CONSTITUTION: When the printing control circuit 1 receives data to be printed hereafter, it discriminates whether a fixed dots number, for example the number of black dots to be printed of not less than 608 dots exist or not. If under 608 dots exist and the voltage of thermal head driving power source 5 measured with the voltage detection circuit 4 has a returned to a setting voltage value, for example 19V, printing operation is carried out according to the specified printing period T by inputting a driving start command to the thermal head 3 from the head driver circuit 2. Further, when the received dot data amount is not less than 608 dots, the thermal head is divided into left and right two segments and printing is carried out. This prevents the decrease of printing quality due to the difference of print density by making the momentary drop of thermal head driving electric power source 5 not exceed printing density guarantee drooping voltage Δe.