To enhance accuracy of a rotating position by further suppressing generation of backlash.

The index disk is provided with an input shaft 2, an output shaft 3, and a speed-reduction mechanism 1-1 interposed between the input shaft 2 and the output shaft 3. The speed-reduction mechanism 1-1 is a planetary roller mechanism 1. Backlash is effectively reduced and index accuracy is remarkably improved in the planetary roller mechanism 1. Among other things, the index disk is applied as the index disk for manufacturing a gear which has no backlash and is excellent in performance of a power transmission mechanism and as a machine tool to index the rotating position by controlling remarkably and with high accuracy. The index disk without using the gear in order to manufacture a high-performance gear is applied most suitably as a mother machine for manufacturing the gear.