To effectively prevent an on-vehicle battery from being overcharged at the time of going downhill, by decelerating the traveling speed with a decelerating means if a detected battery voltage is judged to exceed a first given value.

If a battery voltage is judged to exceed a given value, an electric motor 28 is used as a generator when traveling in deceleration on a downhill, for instance, and a battery 38 is charged while the traveling speed is decelerated with regenerative braking. But, charging voltage increases on a long downhill. If the battery voltage exceeds an overcharged voltage, therefore, duty ratio is made to decline gradually. Specifically, the duty ratio is made to decrease continuously by 1%, for example, so that the traveling speed decreases by about 2 km/h during the given period of time. This structure can effectively prevent an on-vehicle battery from being overcharged at the time of going downhill. The car does not have to be stopped. Simply needed is the decrease of the traveling speed.