To improve safety driving by making an auxiliary wheel free to lift off and ground, extending a distance between the left and right auxiliary wheels at the time of grounding and contracting it to the minimum at the time of revolving and pulling it up.

In case of revolving and pulling a grounded auxiliary wheel 6 up, a wire 13 pulls up a hooking apparatus, a hooked apparatus and a wire 8, an auxiliary wheel support 3 starts to be revolved and pulled up, the hooked apparatus further rises and pushes out an unfixing lever 9, the wire 8 gets on the lever returned to its starting point by a spring and does not drop, and the auxiliary wheel 6 is fixed in a stored state. When the lever 18 is released, the hooking apparatus and the lever 18 return to their starting points by a pressing spring installed on the hooking apparatus, but the auxiliary wheel 6 comes to be in the stored state. In the case of grounding the auxiliary wheel 6, the unfixing lever 9 is pressed and it turns around a shaft 11 as its center, its head end goes out of a fixing device 12, the hooked apparatus is unfixed, it revolves and drops by gravity of the auxiliary wheel, etc., and work of a pulling spring 5, enlarges an interval between the auxiliary wheels and grounds.