A motion conversion mechanism includes: a cylindrical body having an internal screw and third gear teeth in an inner periphery thereof, the internal screw being provided in a screw region that is an intermediate region in an axial direction of the cylindrical body; a shaft body disposed coaxially with the cylindrical body inside the cylindrical body and having first gear teeth and a first external screw in an outer periphery thereof; and a plurality of planetary rollers. First planetary rollers among the planetary rollers mesh with the gear teeth formed in a first gear region and do not mesh with the gear teeth formed in a second gear region, and second planetary rollers among the planetary rollers mesh with the gear teeth formed in the second gear region and do not mesh with the gear teeth formed in the first gear region.