A linear motion actuator (1,100) comprising: a ball screw device (2,110) having a screw shaft (7,112), a nut (8,111,111G), and a plurality of balls (9,113); a piston (4,120,120A,120B,120C,120D,120E,120F,120G,120H) mounted on one end of the screw shaft (7,112); and a stroke limiting mechanism (3,140) setting an operation starting point in time of the screw shaft (7,112) toward a first direction pointed by the one end, the nut (8,111,111G) having: one end face directed to the first direction; and a protruding part (15,117,117G) protruding from the one end face, the piston (4,120,120A,120B,120C,120D,120E,120F,120G,120H) having: an opposite face directed to a second direction, which is opposite to the first direction, and opposite the one end face, characterized by a clearance groove part (18,127,127A,127B,127C,127D) recessed from the opposite face in the first direction and extending in a rotational direction about the screw shaft (7,112); and a stepped face placed on an end of the clearance groove part (18,127,127A,127B,127C,127D) in one of the rotational direction out of wall faces surrounding the clearance groove part (18,127,127A,127B,127C,127D) and directed to another of the rotational direction, and the protruding part (15,117,117G) and the stepped face are in contact with each other to form the stroke limiting mechanism (3,140). a ball screw device (2,110) having a screw shaft (7,112), a nut (8,111,111G), and a plurality of balls (9,113); a piston (4,120,120A,120B,120C,120D,120E,120F,120G,120H) mounted on one end of the screw shaft (7,112); and a stroke limiting mechanism (3,140) setting an operation starting point in time of the screw shaft (7,112) toward a first direction pointed by the one end, the nut (8,111,111G) having: one end face directed to the first direction; and a protruding part (15,117,117G) protruding from the one end face, one end face directed to the first direction; and a protruding part (15,117,117G) protruding from the one end face, the piston (4,120,120A,120B,120C,120D,120E,120F,120G,120H) having: an opposite face directed to a second direction, which is opposite to the first direction, and opposite the one end face, characterized by a clearance groove part (18,127,127A,127B,127C,127D) recessed from the opposite face in the first direction and extending in a rotational direction about the screw shaft (7,112); and a stepped face placed on an end of the clearance groove part (18,127,127A,127B,127C,127D) in one of the rotational direction out of wall faces surrounding the clearance groove part (18,127,127A,127B,127C,127D) and directed to another of the rotational direction, and an opposite face directed to a second direction, which is opposite to the first direction, and opposite the one end face, characterized by a clearance groove part (18,127,127A,127B,127C,127D) recessed from the opposite face in the first direction and extending in a rotational direction about the screw shaft (7,112); and a stepped face placed on an end of the clearance groove part (18,127,127A,127B,127C,127D) in one of the rotational direction out of wall faces surrounding the clearance groove part (18,127,127A,127B,127C,127D) and directed to another of the rotational direction, and the protruding part (15,117,117G) and the stepped face are in contact with each other to form the stroke limiting mechanism (3,140).