In the case where a virtual vehicle speed selection means selects one of a first virtual vehicle speed and a second virtual vehicle speed so as to perform switching from the other one to said one of the first virtual vehicle speed and the second virtual vehicle speed, the rotation speed of an electric motor is decelerated to a threshold value with which travel of the vehicle stabilizes and the deceleration of the electric motor is stopped when the rotation speed of the electric motor becomes lower than the threshold value, and then traction of the vehicle is controlled based on the selected virtual vehicle speed.