PURPOSE: To obtain almost equal printing grade with respect to many printing fonts, by supplying a current to the corresponding heat generator for a period, which is shorter than a period supplying a current to the heat generator corresponding to a printing dot, even with respect to a non-printing dot.

CONSTITUTION: A current is supplied to the heat generating resistors 2-1W2-n corresponding to printing dots and a current is also supplied even to the heat generating resistor corresponding to a non-printing dot for a period shorter than a period supplying the heat generating resistors corresponding to the printing dots. In each printing timing, not only the supply of a current for obtaining heat energy required in printing is performed to each printing dot but also a current is supplied even to the heat generating resistor corresponding to the non-printing dot for a period shorter than the current supply time to the heat generating resistor corresponding to each printing dot to always heat each heat generating resistor. Therefore, the quantity of heat used for the printing of a printing dot can be kept constant.