PURPOSE: To form miniature and simple structure and reduce backlash even at the time of a high deceleration ratio in a decelerator.

CONSTITUTION: Three disks 1, 4, and 8 have spiral grooves 18 wherein a ratio between the increments of the radius and circumferential angles of the disks are made constant. Plural rolling elements 7 and 5 interposed in the grooves of facing disks 1 and 4 also 4 and 8 are retained in fixed positions in the circumferential directions of the disks 1, 4, and 8 by a guide 6. Decelerating is performed by the combination of the ratios of the increments of the radius and circumferencial angle of the respective disks in the grooves of the facing disks 1 and 4 also 4 and 8.