PURPOSE: To provide a circuit that a simple structure and requires less time to checking bits of a thermal head.

CONSTITUTION: With a logic switch 1b for normal printing operation turned off and a logic switch 1a for checking turned on, a check circuit 6 is set in a state of executing checking, and voltage Va obtained by dividing the voltage from a thermal head 3 by a voltage-dividing resistor 2 is compared in an error- detecting circuit 61 with a specified voltage. Thereby checking such as detecting deteriorations in heat-generating bodies 5 or disconnection of wires in the thermal head can be executed.