PURPOSE: To prevent possible change in property of following topography of a ground sensor when a vehicle advances and retracts, by moving the ground sensor so that a relative distance is kept constant between the ground sensor and a caster wheel.

CONSTITUTION: A caster 30 has a caster wheel 31 and a caster arm 32 supporting it and supported free to turn with respect to an arm 20 centered on vertical axis line thereof through a crank shaft 51 connected directly at the upper part of the caster arm 32. The caster wheel 31 is arranged with a ground point thereof offset by trail value (t) as for a spiral shaft of the caster 30. On the other hand, a crank shaft 51 is housed into a casing 20A formed at a free end of the arm 20 and a crank radius (r) of the crank shaft 51 is set equal to the trail value (t) of the wheel 31. Then, the ground sensor 40 is moved so that a relative distance is kept constant between the ground sensor 40 supported on the arm 20 and the wheel 31.