PURPOSE: To reduce uneven density of an image by controlling the drive current level or the driving time according to the resistance value of heat generating resistor.

CONSTITUTION: The sequential transfer of electric charges to capacitors 5a, 5b... is controlled sequentially through trasistors 6a, 6b... by way of a system control section 12 to accumulate a voltage into capacitors 5a, 5b... according to the resistance value of heat generating resistor 1 memorized in the control section 12. According to the voltage thus accumulated, a switching transistor 2 is controlled through an AND gate 7 and a buffer 3 responding to a decoded signal from a decoding section 11 on stages of a shift register 8 and a control signal from the control section 12 so that the drive current level of the resistor 1 varies with the resistance value. Thus, the resistors varied in the resistance value are heated to be uniform in the temperature according to selection made thereby reducing the uneven density of images. The same can be done by controlling the current driving time according to the resistance value.