Abstract: Such a situation that excessive loads repetitively act on a force sensor and a robot arm and an excessive load acts on each work piece is suppressed, and even if there is a variation in a mass of a first work piece gripped by a robot hand, a high-precise fitting operation can be executed. A control unit for impedance-controlling the operation of a robot arm so that one work piece is come into contact with another work piece moves a robot hand for gripping the one work piece, and sets a value acquired before the start of the impedance control to a first target value. The impedance control is made based on the first target value.