An actuator comprising: a first piston, for high load, inserted into a housing so as to be movable in a direction of an axis; a second piston, for low load, movably inserted into a cylindrical hole extending through the first piston in the direction of the axis, the second piston having, at a leading end portion thereof, an output portion projecting to an outside of the housing; a pressure fluid chamber and a spring chamber formed by partitioning an inside of the housing by the first piston and the second piston; a first spring, for high load, mounted in the spring chamber so as to urge the first piston toward the pressure fluid chamber side and received by a wall surface of the spring chamber; characterised by a second spring, for low load, mounted in the spring chamber so as to urge the second piston toward the pressure fluid chamber side, the second spring having a lower end received by a lower wall of the housing; and a switching mechanism having a plurality of engagement members arranged along an outer periphery of the second piston so as to be spaced apart from each other at predetermined intervals in a circumferential direction, the switching mechanism being disposed in the pressure fluid chamber so as to connect the first piston selectively to the housing or the second piston via the engagement members, wherein the switching mechanism includes: an outer peripheral straight surface and an engagement groove provided in an outer peripheral surface of the second piston in order from the pressure fluid chamber side toward the spring chamber side; a first pressing portion provided in the first piston so as to press the engagement members toward the pressure fluid chamber side; a second pressing portion provided in a peripheral wall of the pressure fluid chamber so as to receive therein the engagement members that are in a state of being in contact with the outer peripheral straight surface, the second pressing portion engaging the engagement members with the engagement groove when the second piston advances toward the pressure fluid chamber side; and an inner peripheral straight surface provided in the peripheral wall of the pressure fluid chamber so as to come into contact with the engagement members that are in a state of being engaged with the engagement groove, and the switching mechanism is configured such that the first piston is connected to the second piston via the engagement members and the connection between the first piston and the housing is released to allow the first piston to move in the direction of the axis relative to the housing when the second spring advances the second piston toward the pressure fluid chamber side by a predetermined stroke in a state where the first piston is connected to the housing via the engagement members, during an actuator operating period when a pressure fluid in the pressure fluid chamber is discharged. a first piston, for high load, inserted into a housing so as to be movable in a direction of an axis; a second piston, for low load, movably inserted into a cylindrical hole extending through the first piston in the direction of the axis, the second piston having, at a leading end portion thereof, an output portion projecting to an outside of the housing; a pressure fluid chamber and a spring chamber formed by partitioning an inside of the housing by the first piston and the second piston; a first spring, for high load, mounted in the spring chamber so as to urge the first piston toward the pressure fluid chamber side and received by a wall surface of the spring chamber; a switching mechanism having a plurality of engagement members arranged along an outer periphery of the second piston so as to be spaced apart from each other at predetermined intervals in a circumferential direction, the switching mechanism being disposed in the pressure fluid chamber so as to connect the first piston selectively to the housing or the second piston via the engagement members, wherein an outer peripheral straight surface and an engagement groove provided in an outer peripheral surface of the second piston in order from the pressure fluid chamber side toward the spring chamber side; a first pressing portion provided in the first piston so as to press the engagement members toward the pressure fluid chamber side; a second pressing portion provided in a peripheral wall of the pressure fluid chamber so as to receive therein the engagement members that are in a state of being in contact with the outer peripheral straight surface, the second pressing portion engaging the engagement members with the engagement groove when the second piston advances toward the pressure fluid chamber side; and an inner peripheral straight surface provided in the peripheral wall of the pressure fluid chamber so as to come into contact with the engagement members that are in a state of being engaged with the engagement groove, and