Provided are a ball-type speed reducer and a rotary table device that are capable of performing high-speed rotation and high-rigidity rotation and suitable for reduction in size and weight. An index table 10 includes a ball-type speed reducer 20 and a table 50 that is fixed to an upper surface of a worm wheel 30 of the ball-type speed reducer 20. When the worm gear 22 rotates as the input shaft 24 of the ball-type speed reducer 20 rotates, balls 36 made of a magnetic material move along a ball groove 26 having a spiral shape. The worm wheel 30 rotates as the balls 36 move, and the table 50 rotates. The balls 36 are attracted and held by the permanent magnet 34 on the worm wheel 30 side, so that a case for preventing the balls 36 from coming off the worm wheel 30 can be omitted, whereby the total size and weight can be reduced. Backlash is not required, so that high-speed rotation and high-rigidity rotation are possible.