A linear motion mechanism, comprising: an electric motor; a ball screw which is rotationally driven around an axis by the electric motor; a nut which is screwed into the ball screw, and advances and retreats relative to the ball screw in an axial direction of the ball screw according to rotation of the ball screw; a casing which surrounds the ball screw and the nut; a tubular cylinder rod which includes a base end section which is connected to the nut inside the casing and a tip section which is exposed to the outside of the casing, and into which the ball screw can be inserted; and a nut-side grease supply hole which is formed in the nut and extends in the direction along a center axis of the ball screw, wherein a first end of the nut-side grease supply hole is a discharge port opening toward an outer circumferential surface of the ball screw and the second end of the nut-side grease supply hole opens to a surface of a first end side of the nut, characterized by further comprising: a cylinder rod-side grease supply hole which is formed in the cylinder rod, wherein a first end of the cylinder rod-side grease supply hole is an inlet opening to an outer circumferential surface of the cylinder rod and exposed to the outside of the casing, and a second end of the cylinder rod-side grease supply hole opens to a flanged portion of the cylinder rod, an opening on the second end of the cylinder rod-side grease supply hole and an opening on the second end of the nut-side grease supply hole are connected so as to face each other, and the cylinder rod-side grease supply hole communicates with the nut-side grease supply hole. an electric motor; a ball screw which is rotationally driven around an axis by the electric motor; a nut which is screwed into the ball screw, and advances and retreats relative to the ball screw in an axial direction of the ball screw according to rotation of the ball screw; a casing which surrounds the ball screw and the nut; a tubular cylinder rod which includes a base end section which is connected to the nut inside the casing and a tip section which is exposed to the outside of the casing, and into which the ball screw can be inserted; and a nut-side grease supply hole which is formed in the nut and extends in the direction along a center axis of the ball screw, wherein a first end of the nut-side grease supply hole is a discharge port opening toward an outer circumferential surface of the ball screw and the second end of the nut-side grease supply hole opens to a surface of a first end side of the nut, characterized by further comprising: a cylinder rod-side grease supply hole which is formed in the cylinder rod, wherein a first end of the cylinder rod-side grease supply hole is an inlet opening to an outer circumferential surface of the cylinder rod and exposed to the outside of the casing, and a second end of the cylinder rod-side grease supply hole opens to a flanged portion of the cylinder rod, an opening on the second end of the cylinder rod-side grease supply hole and an opening on the second end of the nut-side grease supply hole are connected so as to face each other, and the cylinder rod-side grease supply hole communicates with the nut-side grease supply hole. a cylinder rod-side grease supply hole which is formed in the cylinder rod, wherein a first end of the cylinder rod-side grease supply hole is an inlet opening to an outer circumferential surface of the cylinder rod and exposed to the outside of the casing, and a second end of the cylinder rod-side grease supply hole opens to a flanged portion of the cylinder rod, an opening on the second end of the cylinder rod-side grease supply hole and an opening on the second end of the nut-side grease supply hole are connected so as to face each other, and the cylinder rod-side grease supply hole communicates with the nut-side grease supply hole.