To provide a stretch device for regulating of a pulley mounting to shorten a belt exchange work time and reduce working labor.

In a stretch device 6 for regulating mounting of a pulley to regulate the mounting state of a plurality of pulleys to transmit a drive force by belts arranged separately away from each other, a stretch body 61 and a parallelism central position regulating unit 62 are provided. One end of the stretch body 61 makes contact with the side of a pulley 12 on the drive side with a fly wheel 13 located therebetween and the other end part thereof is positioned opposite to the side of a pulley 22 on the driven side and forms a reference surface S paralleling the side of the pulley 12 on the drive side. The parallelism central position regulating unit 62 sets the side of the pulley 22 on the driven side in parallel to the side of the pulley 12 on the drive side and the belt central position of the pulley 22 on the driven side is caused to coincide with the belt central position of the pulley 12 on the drive side.