To provide a method of manufacturing a rotation-linear motion converting mechanism in which at least one of gears fitted to both ends of each planetary gear is formed separately from the body of each planetary shaft and capable of suppressing the inclination of the planetary shaft.

This method of manufacturing the rotation-linear motion converting mechanism 1 comprises steps as follows. The step of assembling a first assembly by combining a sun shaft body 31 with each planetary shaft body 41. The step of fitting a retainer to each planetary shaft body 41 of a first assembly. The step of assembling a second assembly by combining the first assembly with a front ring gear 22. The step of assembling a third assembly by combining the second assembly with a ring shaft body 21. The step of assembling a gear assembly by combining gears 23, 33 and 43 with one another. The step of assembling a fourth assembly by combining the third assembly with the gear assembly.