PURPOSE: To secure stable traveling steerage for both a flat and a stairway part, by making a crawler's lower part grounding surface alterable properly to a flat form and also such a form that has an almost one-point contact floor type projecting part, with a grounding surface altering mechanism.

CONSTITUTION: When a car travels on a flat floor A, both upper and lower rollers 11 and 12 are made to go down as far as an extent of height (h) simultaneously by the drive of a pressure cylinder 18 of a grounding surface altering mechanism 10 via support arms 13, 14 and a connecting rod 15. And, an almost middle part at the lower side of an endless traveling crawler 6 is depressed so as to be projected downward by the lower side roller 12. Likewise, in time of traveling up and down on a stairway part, with the drive of the pressure cylinder 18 of the lower part grounding altering mechanism 10, these upper and lower rollers 11 and 12 are made to go up as far as an extent of the height (h) simultaneously via these support arms 13, 14 and the connecting rod 15.