To provide a linear motion mechanism suitable for slight feed, having a few number of parts, and capable of reducing a size.

An input shaft 11 is supported by a housing 13 rotatably around a certain axis and not movably in an axial direction. An output shaft 12 is supported by the housing 13 rotatably around a certain axis and not movably in an axial direction. The input shaft 11 has a screw 11a integrally and has a gear 11b integrally. The output shaft 12 has a screw 12a meshing with a screw 11a of the input shaft 11 integrally and the gear 12b meshing with the gear 11b of the input shaft 11 integrally. The output shaft 12 moves in the axial direction while rotating, and the gear 12b of the output shaft 12 moves in the axial direction relative to the gear 11b of the input shaft 11 while keeping the meshing with the gear 11b of the input shaft 11 by a motor 15 rotating the input shaft 11.