While a voltage value of the battery detected by a battery voltage-detecting part is less than a predetermined threshold voltage value for determining a low voltage state, a rotation sensor malfunction-detecting part stops calculating a vehicle speed and uses the vehicle speed calculated at a time immediately before a time when a voltage value of a battery becomes less than the threshold voltage value for determining the low voltage state, for detecting malfunction in a rotation sensor. Thereby, even if noises due to a voltage fluctuation in the battery mix with the rotation signal from the rotation sensor, since the rotation sensor malfunction-detecting part does not calculate the vehicle speed during a period of time when the voltage value of the battery detected by a battery voltage-detecting part is less than the threshold voltage value for determining the low voltage state, the vehicle speed is not miscalculated based on the rotation signal including noises, and the malfunction in the rotation sensor can be precisely detected.