PURPOSE: To eliminate irregularlity in density, trailing or the like from occurring and contrive to enhance print quality, by controlling the current-passing time for heating resistors in accordance with the temperatures detected by temperature sensors.

CONSTITUTION: The current-passing time (t) for the heating resistor is controlled in accordances with the temperature Ts detected by the nearest one of the temperature sensors 5AW5E so that t=t5 when Ts≤T1, t=t4 when T1<Ts≤T2, t=t3 when T2<Ts≤T3, t=t2 when T3<Ts≤T3, t=t1 when T4<Ts≤T5, and t=0 (passing no current) when T5<Ts. For example, the heating resistor is divided into 10 sections 1st common (a)W10th common (j), and when the temperatures Ts detected by the temperature sensors 5AW5E are T1<Ts(5A)<T2, T2<Ts(5B)<T3, T3<Ts(5C)<T4, T2<Ts(5D)<T3 and Ts(5E)<T1, the current- passing times for the commons of the thermal head may be set to be as shown. In this case, the current-passing time for each of 2nd, 4th, 7th and 9th commons located at an intermediate part between an adjacent pair of the temperature sensors is set in accordance with the lowest of the temperatures detected by the pair of the temperature sensors.