PURPOSE: To enhance accuracy of the longitudinal position between support trucks and prevent abrasion by providing each one ball bearing on each one end of respective support trucks, bringing two support trucks in contact with each other through each ball bearing, and bringing the support trucks and a rail into contact with one another by a roller pressed to the rail by an elastic means.

CONSTITUTION: Respective support trucks 2B, 2C have ball bearings 26 or roller bearings on their one end, and two support trucks are brought into contact with each other through the ball bearings 26 or the roller bearings. Respective support trucks are brought in contact with a rail 1 over the whole closed track, and the support trucks and the rail 1 are brought in contact with one another by rollers 19, 23 either of which is pressed against the rail 1 by an elastic means 25. Since the ball bearings 26 have little backlash, they can bring two support trucks 2B, 2C in direct-contact with each other, and therefore, positioning in the longitudinal direction between support trucks can be accurately performed, and abrasion of the support trucks is prevented.