A left drive shaft and a right drive shaft each include an inside constant velocity joint and an outside constant velocity joint with a ball spline mechanism provided between the inside constant velocity joint and the outside constant velocity joint for enabling the distance between the inside constant velocity joint and the outside constant velocity joint to be contracted and extended. The ball spline mechanism includes a spline shaft having an outer circumferential surface provided with a plurality of axial grooves extending in the axial direction, a tubular portion surrounding the periphery of the spline shaft and having an inner circumferential surface provided with a plurality of axial grooves extending in the axial direction and a plurality of balls capable of rolling while being fitted in the axial grooves and in the axial grooves. In addition, a left lower arm and a right lower arm are formed into an A-shape and are supported by a lower part of a rising portion and a lower end of the downwardly extending portion via a lower supporting shaft.