An actuator comprising: a cylinder supported by a supporting plate together with a drive motor ; a piston located reciprocatably in the cylinder ; a ball screw shaft located in the cylinder and driven by the drive motor ; a slide block screwed with one end of the ball screw shaft in the cylinder and coupled to an end of the piston ; a first linear motion bearing unit and a second linear motion bearing unit each of which is located in a bearing housing at one end of the cylinder and has a rail provided on the piston to extend along a central axis of the piston and a guide block configured to support the rail via a rolling element and which support the piston movably; and a third linear motion bearing unit and a fourth linear motion bearing unit each of which is located in the cylinder and has a rail pedestal provided on an inner peripheral surface of the cylinder to extend along the central axis of the piston and a guide block configured to support the rail pedestal via a rolling element and which support the slide block movably together with the piston , wherein the first linear motion bearing unit and the second linear motion bearing unit are located individually in a position in a circumferential direction of an outer peripheral portion of the piston in which a weight of the piston is applied and a position in a direction orthogonal to the direction in which the weight of the piston is applied, respectively, and the third linear motion bearing unit and the fourth linear motion bearing unit are located individually in a position in a circumferential direction of an outer peripheral portion of the slide block in which the weight of the piston is applied and a position in a direction orthogonal to the direction in which the weight of the piston is applied, respectively. a cylinder supported by a supporting plate together with a drive motor ; a piston located reciprocatably in the cylinder ; a ball screw shaft located in the cylinder and driven by the drive motor ; a slide block screwed with one end of the ball screw shaft in the cylinder and coupled to an end of the piston ; a first linear motion bearing unit and a second linear motion bearing unit each of which is located in a bearing housing at one end of the cylinder and has a rail provided on the piston to extend along a central axis of the piston and a guide block configured to support the rail via a rolling element and which support the piston movably; and a third linear motion bearing unit and a fourth linear motion bearing unit each of which is located in the cylinder and has a rail pedestal provided on an inner peripheral surface of the cylinder to extend along the central axis of the piston and a guide block configured to support the rail pedestal via a rolling element and which support the slide block movably together with the piston , wherein the first linear motion bearing unit and the second linear motion bearing unit are located individually in a position in a circumferential direction of an outer peripheral portion of the piston in which a weight of the piston is applied and a position in a direction orthogonal to the direction in which the weight of the piston is applied, respectively, and the third linear motion bearing unit and the fourth linear motion bearing unit are located individually in a position in a circumferential direction of an outer peripheral portion of the slide block in which the weight of the piston is applied and a position in a direction orthogonal to the direction in which the weight of the piston is applied, respectively. <EOS>