To provide a shaft fitted with a gear capable of suppressing favorably the deformation of the gear after being mounted while the relative rotation of the gear and shaft is suppressed, and to provide a rotational-linear motion converting mechanism using the shaft described.

The rotational-linear motion converting mechanism keeps a peripheral surface equipped with a sun shaft 10 having a spur gear 12. A collar 50 is set between the peripheral surface of the sun shaft 10 and the inside surface of the spur gear 12. A first projection 10b formed of a material harder than the collar 50 is formed on the peripheral surface of the sun shaft 10 whereto the collar 50 abuts. The inside circumferential surface of the spur gear 12 whereto the collar 50 abuts has a second projection 12b formed of a material harder than the collar 50.