To simplify the constitution and to prevent transmission of a load fluctuation and deformation of a timing belt due to no use for a long time.

An arm 45 is rotatable around a spindle 49 between a driving pulley 39 and a driven pulley 41. An arm 45 is provided with tension pulleys 46 and 47 with the spindle 49 nipped therebetween, and a tension spring 50 is hooked on one end side. By energizing the arm 45 in the rotational drive direction of a timing belt 40 by the tension spring 50, the tension pulley 46 is pressed against the tension side 40a and the tension pulley 47 against the slack side 40b respectively. Further, during drive, the arm 45 is further rotated in an energizing direction to increase press of the tension pulleys 46 and 47. When a load is increased, the tension pulleys 46 and 47 are escaped and a load fluctuation is absorbed.