PURPOSE: To automate the adjustment of tension and maintain proper tension for a long period in a transfer mechanism for reciprocating a movable body with a belt stretched over between a pair of pulleys by connecting one end of a toothed belt directly to the movable body and the other end to the movable body through a one-way clutch and tension spring.

CONSTITUTION: A toothed belt 3 is prevented from a sag by the operation of a one-way clutch 8 even if large tension is exerted on the toothed belt 3. If the belt 3 is extended, the extended belt 3 is pulled by a tension spring 4 which absorbs the elongation. Thus, the tension is automatically adjusted while proper tension is maintained for a long period.