PURPOSE: To simplify the power source using a smoothed non-stabilization power source, by monitoring the voltage fed for recording at a thermal head section to control the width of a pulse applied so that the squared detected voltage multiplied by applied pulse width of a detection voltage becomes almost constant.

CONSTITUTION: Voltage in an AC line is lowered with a transformer 11, full- wave rectified with a rectifying diode 12 and smoothed with a capacitor 13 to be introduced into a thermal head 20 with lines l2 and l2'. The voltage (a) monitored with terminals VHD'2 and GND'2 is inputted into a pulse generator 30, which generates a pulse (c) with the pulse width according to the voltage by an SB trigger signal (b) to drive a heat generating body R. The pulse generator 30 is controlled so that the mean value of the voltage and the pulse width within a predetermined time will meet the requirement of V12.tPWH1=V22.tPWH2= V32.tPWH3....