To provide an inner circulation type ball screw whose durability can be enhanced by averaging thickness of wall surfaces between circulating passages.

A ball screw includes a screw 11, a nut, a circulating unit 13 and a plurality of rolling elements. The nut fitted to the screw 11 has a chamber in the inner surface. The circulating unit 13 is arranged in the chamber, and has a first circulating passage and a second circulating passage. The first circulating passage has a first prestage part, a first curved part 134, and a first poststage part, and the second circulating passage has a second prestage part, a second curved part 137, and a second poststage part. The first curved part 134 and the second curved part 137 are distributed along the screw 11, and are alternately arranged in a circumferential direction of the screw 11 to form an angle α. The plurality of rolling elements roll between a first thread, a second thread, the first circulating passage and the second circulating passage.