PURPOSE: To detect the breaking of wire before starting, by providing a means of flowing a constant current to an ammeter when a power voltage is applied to an engine rotation display driving circuit which flows the current corresponding to a rotational frequency to the ammeter.

CONSTITUTION: Between a power source Vcc and an earth point, an ammeter 3 for displaying the rotational frequency of an engine and a transistor (TR) 2 are conncted. A pulse of an input signal In which corresponds to the engine rotational frequency is supplied to an input terminal 1 to flw a current proportional to the frequency of the signal In to the ammeter 3. The output of a constant voltage circuit 7 is applied to the base of a TR5 connected in parallel to the TR2. When the power source is put to work, the constant current flows through the TR5 unless the wire is broken. Thus, the breaking of the wire is detected before the engine is started.