An actuator that converts a direction of action of driving force between a rotational direction and a linear direction to drive equipment, the actuator comprising: a rotational drive portion that generates or outputs rotational driving force; a linear drive portion that outputs linear driving force when the rotational drive portion generates the rotational driving force, or generates linear driving force when the rotational drive portion outputs the rotational driving force; a piston integrated with, or fixed to, the linear drive portion; a driving force conversion mechanism that has a nut portion and a screw portion, and converts a direction of action of the driving force between a rotational direction and a linear direction and transmits the driving force as a result of relative rotation of the nut portion and the screw portion; a case that has a tubular portion and within which the piston and the driving force conversion mechanism are installed; and a piston sliding portion that is provided within the case and fixed to the case, and has an inner circumference slidably supporting an outer circumference of the piston, wherein one of the nut portion and the screw portion is provided so as to be displaced with the piston, wherein the other of the nut portion and the screw portion is provided so as to rotate with the rotational drive portion, or rotate in conjunction with the rotational drive portion via a rotational driving force transmission mechanism, wherein the piston is installed so as to be able to slide in an axial direction of the case with respect to the piston sliding portion, wherein the piston has a tubular portion and internally houses the driving force conversion mechanism, and wherein an axis of the tubular portion of the piston and a central position of the inner circumference of the piston sliding portion are set to be eccentric with respect to an axis of the screw portion and of the nut portion, characterized in that an axis of the linear drive portion is arranged coaxially with said axis of the screw portion and of the nut portion. a rotational drive portion that generates or outputs rotational driving force; a linear drive portion that outputs linear driving force when the rotational drive portion generates the rotational driving force, or generates linear driving force when the rotational drive portion outputs the rotational driving force; a piston integrated with, or fixed to, the linear drive portion; a driving force conversion mechanism that has a nut portion and a screw portion, and converts a direction of action of the driving force between a rotational direction and a linear direction and transmits the driving force as a result of relative rotation of the nut portion and the screw portion; a case that has a tubular portion and within which the piston and the driving force conversion mechanism are installed; and a piston sliding portion that is provided within the case and fixed to the case, and has an inner circumference slidably supporting an outer circumference of the piston, wherein one of the nut portion and the screw portion is provided so as to be displaced with the piston, wherein the other of the nut portion and the screw portion is provided so as to rotate with the rotational drive portion, or rotate in conjunction with the rotational drive portion via a rotational driving force transmission mechanism, wherein the piston is installed so as to be able to slide in an axial direction of the case with respect to the piston sliding portion, wherein the piston has a tubular portion and internally houses the driving force conversion mechanism, and wherein an axis of the tubular portion of the piston and a central position of the inner circumference of the piston sliding portion are set to be eccentric with respect to an axis of the screw portion and of the nut portion, characterized in that an axis of the linear drive portion is arranged coaxially with said axis of the screw portion and of the nut portion.