PURPOSE: To obtain an exact input/output relation, by starting a charge before the discharge of a capacitor is executed sufficiently, when input pulse frequency exceeds some value, and making an output voltage level constant so that output voltage does not drop suddenly.

CONSTITUTION: In case when input pulse frequency is low, an F/F53 is set by a trigger pulse, and a capacitor 55 is charged by a charging current supplying circuit 54. When output terminal voltage V0 attains to reference voltage V1, a reset pulse is supplied from a comparator 58 and the F/F53 is reset. When the voltage V0 becomes lower than reference voltage V2 as the capacitor 55 is discharged, an output level of a comparator 59 is inverted. When an output of the comparator 59 is "1", a transistor 63 is turned on by an output of a comparator 62, a current flows to a tachometer 65, and display (an engine speed) is executed. When the input frequency exceeds some value, charging is started before the voltage V0 becomes lower than the reference voltage V2, an output of the comparator 59 remains "1", and the tachometer 65 displays constant engine speed.