PURPOSE: To easily prevent a belt from slipping aside by providing rollers free to rotate against an axle on both edge parts of at least more than one of slave rollers except for a drive roller and gradually increasing the outer diameter of the rollers as they approach the edges of the axle.

CONSTITUTION: Rollers 2, 3 rotatable about an axle 1 are provided on both edge parts of the axle 1 of at least more than one of slave rollers 13 except for a drive roller 12. At this time, the outer diameter of the rollers 2, 3 is gradually increased as they approach the edge of the axle 1. Consequently, when a belt 11 slips out of the center in the direction of the axle 1 and slips aside, the edge part of the belt 11 runs on the periphery of the rollers 2, 3, gradually increasing on the edge of the axle 1, and the belt 11 is pushed back to the central part in the direction of the axle 1.