To prevent generation of the excess load in rotation-linear motion conversion even when a planetary differential screw type rotation-linear motion conversion mechanism member wherein a gear component is fitted into a corresponding portion is used.

After a spur gear component 28 is gap-fitted to a sun shaft body 11, key grooves of the spur gear component 28 and the sun shaft body 11 are made to be coincident with each other, and are positioned by a key 30. The spur gear component 28 is completely fixed to the sun shaft body 11 by caulking in this state, and thereby the completed spur gear part is arranged in a prescribed phase position accurately with respect to a screw part 10a. When the planetary differential screw type rotation-linear motion conversion mechanism is assembled in a control shaft axial position adjusting device with the usage of the sun shaft, suitable meshing can be achieved between members, especially between the sun shaft and a planetary shaft. Therefore, generation of the excess load can be prevented in rotation-linear motion conversion of the planetary differential screw type rotation-linear motion conversion mechanism.