PURPOSE: To greatly improve step-difference running over capacity by providing a travelling wheel to travel on a flat ground and a large diametral semicircular and bow-shaped auxiliary wheel to climb up and down a step-difference and by climbing up and down on the step-difference with the auxiliary wheel.

CONSTITUTION: An auxiliary shaft 33 of a semicircular and bow-shaped auxiliary wheel 34 is supported by a revolving main frame 20 with a leg part to support an axle 31 of a travelling wheel 30 axially installed on an installation base body 10 and a revolution constraint frame 40 capable of oscillating between the main frame 20 and the installation base body 10, and a brake 42 with a projection and a brake receiver 44 are respectively provided between the main frame 20 and the installation base body 10. The auxiliary wheel 34 is in the state floating above the ground and is constituted to be energized to rotate forward by an extension spring extensively provided between the leg part and the auxiliary wheel 34, and the flat ground is travelled on by the travelling wheel 30, and in the case when a corner of a step-difference makes contact with the auxiliary wheel 34, the brake 42 works and constrains revolution, and the step-difference is climbed up and down by the auxiliary wheel 34.