PURPOSE: To minimize resistance in the horizontal movement of a running body by keeping a running range constant with respect to the number of revolutions of a drive wheel.

CONSTITUTION: A running wheel 31 is synchronously driven so that a barrel- shaped bearing 21 of at least one other running wheel 31 contacts a running plate 34 when a mating gear part 3c of one running wheel 31 is meshed with a vertical rack 34 while an outer periphery 21b of the barrel-shaped bearing 21 and a pitch circle 13e of an involute gear 13c of the mating gear part are positioned on the same circle centering a rotating shaft 19 of the running wheel 31.