A linear motion mechanism (27) is provided with: a cylinder rod (312) into which a ball screw (30) can be inserted, said cylinder rod (312) comprising a base end section that is connected to a nut (311) within a piston casing (36) and a tip section (312a) that is exposed on the outside of the piston casing (36); a nut-side grease supply hole (321) that is formed in the nut (311) and that comprises a discharge port (323) that opens toward the outer circumferential surface of the ball screw (30); and a cylinder rod-side grease supply hole (322) that is formed in the cylinder rod (312), that comprises on one end thereof an inlet (324) that opens at a position that is exposed to the outer section of the piston casing (36), and that comprises another end (322b) that is connected to the nut-side grease supply hole (321).