PURPOSE: To improve the over-riding ability against an obstacle and the steering performance, by providing a wheel at the tip of a rotary arm of a travelling body which can ascende and descend a staircase in a reactor container, etc. then rotating said wheel and the rotary arm body independently.

CONSTITUTION: An arm 105 is cylindrical to rotate around the central axis by means of a bearing 121. The arm 105 is provided with a gear 122 to be geared with an intermediate gear 123 supported by a rotary shaft 104, while said gear 123 will be geared with a gear 126 fixed to a rotary shaft 125. Upon rotation of a drive motor 124 to rotate the arm 105, a wheel 106 provided at the tip of the arm 105 will rotate around the central axis of the arm 105. Consequently the grounded wheel 106 is tilted to the left and right direction while to change the direction of said wheels 106. The difference between the inner and outer wheels of the left and right wheels 106 due to said steering is adjusted by a travel controller 128.