PURPOSE: To print while constantly maintaining the heating temperature of a printing head at a substantially fixed value and enable to make uniform printed density, by compensating for the variation of the heating condition of the printing head generated with the progress of printing from the start of printing.

CONSTITUTION: When a printing timing PT is inputted into a register 14 and a timing-generating circuit 16, predetermined printing data PD are printed in dots by the printing head 22 in a heating condition according to a heating pulse signal HT. The printing data PD are converted into heating data HD1, HD2 by weighting circuits 10a, 10b, printing progress data ND based on the printing data PD stored in the register 14 are added to the heating data by an adding circuit 11, data according to the amount of heat accumulated in the head 22 are stored into the register 14 through a multiplexer 15, and the pulse width of the signal HT is reduced in accordance with the heat accumulation. When the printing data PD are a space (blank), the pulse width is increased through a subtaracting circuit 13. Accordingly, the printed density can be made to be uniform.