PURPOSE: To realize accurate measurement of the resistance of thermal head at the stage of shipment of product without requiring a measurement window or the like by a constitution wherein the operations of driver section and a switching means are controlled, data fed from a converting means are subjected to numeric operation and the resistance at each dot of thermal head is detected.

CONSTITUTION: A relay 9 for ON/OFF controlling the current of power supply line COM2, an A/D converter 4 receiving the voltage of the power supply line COM1 in the form of digital signal, a logic control circuit 3 and a relay 9 are combined to constitute four types of circuit relevant to four resistors. A CPU 6 receives combined resistances of respective circuits from the A/D converter 4 and specifies four resistances through numeric operation. Since the resistance at each part can be measured accurately utilizing the function of a driver IC at the stage where a thermal head is assembled, the resistance can be guaranteed at the stage of shipment. Furthermore, no measuring window is required in the measurement of resistance.