PURPOSE: To improve response characteristics without spoiling accuracy in a stationary state by compensating a signal output driving current from a differential comparing circuit when the variation rate of the rotating speed of an engine increases at the time of snapping, etc.

CONSTITUTION: An input signal obtained by detecting the rotation of the engine is inputted to the differential comparing circuit 6 through a frequency-voltage converting circuit 2. When the variation rate of the engine rotating speed is small in normal traveling, the variation rate of the output voltage of the circuit 2 is small and the output value of a differentiating circuit 60 becomes smaller than the reference voltage of an operational amplifier 61. A current flowing through a meter 5, therefore, is only a current flowing through the resistance R0 of a driving circuit 3 and can follow sufficiently up the engine rotating speed. Further, when the variation rate of the engine rotating speed is large at the time of snapping, etc., the variation rate of the output voltage of the circuit 2 is large and the output value of the circuit 60 becomes larger than a reference voltage. A transistor TR2 is therefore turned off to connect resistances R0 and R5 in parallel and the current flowing through the meter 5 becomes larger than when the current flows through only the driving circuit 3, so that meter driving can follow sufficiently up even abrupt variation in the engine rotating speed.