A series elastic actuator includes a first body, a motor, a pulley, a second body, a wire, a first adjuster, a second adjuster, a first spring, and a second spring. The wire is curved around a seat portion of the pulley, a first extending portion of the wire and the first adjuster are elastically supported with respect to the second body by the first spring, and the second extending portion of the wire and the second adjuster are elastically supported with respect to the second body by the second spring. When an external load is applied to the second body and relative rotation is generated between the pulley and the second body, a moment arm by the load is constant and the external load can be measured on the basis of the measured relative rotation angle and the spring constants of the first spring and the second spring.