PURPOSE: To prevent the malfunction of a tachometer by providing a tachometer output circuit dependent upon the battery voltage and the input of engine rotation, a battery abnormality detecting circuit and a switch circuit which stops the tachometer at the time of detecting abnormality.

CONSTITUTION: A tachometer output circuit 3 processes the output signal of a generator 2 based on the voltage of a battery 1 and outputs a tachometer signal corresponding to the rotational frequency of the engine to a tachometer 4 to operate it. A battery abnormality detecting circuit turns off a switch circuit 6 to stop the signal output of the circuit 3 when detecting the voltage abnormality, and the tachometer 4 is stopped. Thus, the malfunction of the tachometer 4 is prevented.