PURPOSE: To reduce the weight of a first arm while the rectilinear function of a second arm is held, by arranging the body of a rotary actuator on a base, and by arranging the center axis of the rotary shaft of a converting mechanism, coaxially with the center of oscillation of the first arm with respect to the base.

CONSTITUTION: The rotary shaft of a converting mechanism 31 is arranged coaxially with the center of oscillation of first arms 14a, 14b, relative to a base. The rotary motions of the output shafts of rotary actuators 16, 30 are transmitted to a converting mechanism 31 from the rotary shaft while the first arms 14a, 14b are oscillated so as to rectilinearly drive a second arm. As a result, it is possible to reduce the weights of the first arms 14a, 14b.