PURPOSE: To prevent the accumulated mileage data from vanishing even in the case of reduced supply voltage by a device wherein the supply voltage is monitored and the mileage data in a counter is transferred to a nonvolatile memory and stored therein when the supply voltage is lowered to the value equal to or less than a predetermined level.

CONSTITUTION: In normal operation where supply voltage is within a predetermined value, pulses from a distance sensor 1 are counted by a counter 2 and accumulated mileage is displayed at an indicator 4 when an ignition switch 5 is turned ON. When the ignition switch 5 is turned OFF, the contents of the counter 2 is stored into a nonvolatile memory 7. On the contrary, with the ignition switch 5 being turned ON once again, the contents of the nonvolatile memory 7 is transferred to the counter 2. When the supply voltage 11b is lowered to the value equal to or less than the normal operation voltage, a voltage detector 11 outputs a detection signal 11c. As a result, an alarm 12 starts to be actuated. In addition, a control unit 6 allows the contents of the counter 2 to be written into the nonvolatile memory 7 immediately after the control unit 6 detects the detection signal 11c.