PURPOSE: To obtain suitable printing density always even though power source voltage is unstable by a method wherein a detection means detecting a pulse width of a pulse signal, a calculation means calculating the pulse width of the pulse signal corresponding to a voltage level, and a means regulating so that a detection pulse width is approximated to a calculation pulse width are established.

CONSTITUTION: A CPU 10a processes power source voltage E to be supplied from a power source Vcc via an A/D converter 24, calculates a pulse width (t1), and emits a specific trigger pulse TS. A pulse width controlling IC 22 adds a regulating pulse TSO having a pulse width (t) to a dot driving circuit 21, and feeds back to the CPU 10a. The CPU 10a detects the pulse width (t) of this regulating pulse TSO and collates by comparison with t1. Then, when t1=t, it is left as it is. When t1≠t, a switch pulse SPi is successively outputted, and a switch Si is ON/OFF controlled. A time constant to be determined as a product of a parallel resistance value of a resistor Ri and a capacity value of a capacitor CB is variably regulated, the detection pulse width (t) is approximated to the calculation pulse width (1).