in fig1 the power of the circuit is supplied from a car battery bt through an ignition switch igs and a fuse fs1 to a voltage stabilizer circuit 14 this voltage stabilizer circuit 14 generates a stable voltage of 5 volts from the power thus supplied and applies the generated voltage to an electronic control unit 10 and an interface circuit 12 in the embodiment the electronic control unit 10 is realized by a micro computer a switch sw1 is a stop switch responsive to manipulation of a brake pedal a switch sw2 is a clutch switch responsive to manipulation of a clutch pedal and as either of these two switches is actuated a constant speed running control process is released or cancelled a switch sw3 is a setting switch for constant speed running that is as this setting switch sw3 is manipulated a given car speed is stored and the constant speed running control process is commenced a switch sw4 is a resume switch for constant speed running that is as this resume switch sw4 is manipulated the constant speed running control process is commenced at the car speed stored before the beginning of the constant speed running control process a switch sw5 is an acceleration switch for increasing the stored car speed being used at the time of constant speed running that is the stored car speed is increased in response to manipulation of this acceleration switch sw5 the value.