PURPOSE: To enable accurate discriminative detection of a short circuit trouble in a plurality of printing head pin drives of a printer, by detecting a short circuit trouble, discriminatively from normal printing, on the basis of an output from the second comparing circuit which compares an output from the first comparing circuit connected to the second power source with the second reference value through a predetermined time constant circuit.

CONSTITUTION: When a transistor 11 is turned ON due to the breakage, or short-circuiting, of some of transistors 11W1N, both voltages VA, VB at points A, B are raised in dependence on the first power source voltage Vcc1 to have waveforms similar to that of the voltage Vcc1. When resistances R1, R2, R3, R4 are so set that VB>VA at the time of such a short circuit, an output from the comparing circuit 12 is brought to a high level. In this case, a voltage Vcc2 has no relation to the voltages at the points A, B. By an output from the circuit 12, the voltage at a connecting point D of the time constant circuit R6, C1 is raised in accordance with the time constant through a resistor R5. The voltage VD at the point D is inputted to a negative terminal of a comparator 13 through a resistor R7, while a reference voltage VE is inputted to a positive terminal of the comparator, and a trouble is detected when VD>VE.