An electric motor is coupled to driving wheels 2, 2. When a slippage of the driving wheels 2, 2 is detected, the electric motor 5 produces a regenerative torque to suppress the slippage of the driving wheels 2, 2. The regenerative torque is controlled in a variable manner according to an index parameter that indicates a road surface condition. Thus, when the slippage of the driving wheels occurs, the slippage of the driving wheels is suppressed according to the road surface condition by a regenerative operation of the electric motor coupled to the driving wheels.