A rotation/linear motion converting mechanism includes a shaft 1 having an external thread 1a on an outer circumferential surface, a nut 2 having an internal thread 2a on an inner circumferential surface, and a roller 3 arranged between the outer circumferential surface of the shaft 1 and the inner circumferential surface of the nut 2. The roller has a thread 3a threaded to the external thread 1a and the internal thread 2a. A spur gear 3b and a spur gear 3c are provided in portions of the roller 3. A spur gear 1b is provided in a portion of the shaft 1. The spur gear 1b is meshed with the spur gear 3b of the roller 3. Internal teeth 4a are provided in a portion of the nut 2. The internal teeth 4a are meshed with the spur gear 3 of the roller 3. This improves the lead accuracy of the shaft 1 when the nut 2 is rotated.