To provide a pulley set capable of selecting installation or non-installation of an interlocking idler, by compensating for the peripheral length between two ring shapes in synchronous transmission, by the ring shapes formed of a specific incomplete round of two pulleys.

A transmission device is constituted by installing a non-slidable synchronous transmission belt between an incomplete round prime mover pulley and an incomplete round driven pulley, and the peripheral length between the ring shapes of two incomplete round pulleys in the synchronous transmission is compensated by the ring shapes formed of two specific incomplete round pulleys, and the incomplete round prime mover pulley also drives the incomplete round driven pulley in the periodic transmission ratio via a synchronous transmission belt, and the length of the synchronous transmission belt installed between the prime mover pulley and the driven pulley is not changed, or a variation is restrained lower than a fastening condition at separation of synchronous transmission belt so that the installation or the non-installation of the interlocking idler can be selected.