A device for checking the printing circuit of a thermal printer wherein pulse currents are passed through only those of the heaters which correspond to printing data stored in a shift register via a power supply resistor and driver transistors to energize the corresponding heaters so as to effect color printing for one operation on heat-sensitive paper. The device comprises a self-examination control section for effecting at least one of the three types of checking, i.e., a check on the driver transistors for malfunction, a check on the heaters for malfunction, and a check on the shift register for malfunction, the arrangement being such that the printing circuit is automatically checked without actual printing operation by making use of the waiting time in which the thermal printer is not printing. Checking the printing circuit by the present device avoids delay in printing and dispenses with manual operation for checking. Thus, it increases the reliability of the thermal printer and avoids wasting heat-sensitive paper and heat-sensitive labels for checking.
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