PURPOSE: To prevent the wear and damage of a bearing and the unexpected rotation under no load, by making constituent wheels slightly nonparallel with each other in the direction of action of load.

CONSTITUTION: Constituent wheels 2 are supported on an axle 1 in such a manner that the wheels are made slightly nonparallel with each other in the direction of action of load upon a caster, namely, in the direction of its forward motion and in the downward direction. The angle of the nonparallelism between the constituent wheels 2 is determined depending on the load upon the caster. Since the wheels 2 are previously made nonparallel with each other, they are made parallel with each other when the load acts to them, so that their rotation is smooth. When the load does not act to the wheels 2, they are nonparallel with each other so that ther rotation is prevented.