An actuator converts the pressure of a fluid into a change in the length of the actuator and is formed such that an elastic tube is spirally wound. The tube is wound around the axis of the actuator. One or more grooves are spirally formed on the outer surface of the tube along the axial center of the tube. When the fluid contained in the inside of the tube is pressurized, a torsional force is applied to the tube along the spirals of the one or more grooves and causes the actuator to contract in the axial direction. Even when an external force acts in a direction in which the actuator is bent, the volume of the inside of the tube is not substantially varied. Accordingly, the actuator can be allowed to freely move in a bending direction.