Abstract: When the device is not in use, supply of power to a computer CPU for controlling the device is automatically interrupted, thus prolonging the service life of the CPU and eliminating useless power consumption. The need of manual operation of a power-on switch can be dispensed with. When at least one of a set switch and a resume switch is actuated, supply of power to the CPU is automatically turned ON using a flip-flop circuit. If the vehicle speed is memorized during an ON state, the CPU is still held in an ON state. But if the vehicle speed is cleared, the CPU controls the flip-flop to set OFF supply of power. In case the vehicle speed is less than a given value, the memorized vehicle speed is cleared irrespective of the state of the set switch and the resume switch.