A first elastic body actuator and a second elastic body actuator are used for driving a first configuration member and a second configuration member to swing them with respect to each other. One end portion of the first elastic body actuator is secured to the first configuration member and the other end portion of the first elastic body actuator is secured to the second configuration member, and one end portion of the second elastic body actuator is secured to the first configuration member and the other end portion of the second elastic body actuator is secured to the second configuration member, such that the position at which the other end portion of the second elastic body actuator is secured to the second configuration member and the position at which the one end portion of the first elastic body actuator is secured to the second configuration member are substantially symmetric about a rotational joint. The interval between the portion of the first elastic body actuator, which is secured to the second configuration member, and the portion of the second elastic body actuator, which is secured to the second configuration member, is moved by a securing-position variable mechanism.