PURPOSE:To rapidly disengage an upper engaging part of a coupling member from an upper coupling member of working machine downward by laying an elastic member between a top link and the coupling member in the direction to enlarge an angle between the top ring and the coupling member. CONSTITUTION:A top link 4 and a lower link 6 pivoted on a body 2 and a coupling member 10 pivoted and connected to the rear of the body constitute a quadric linking mechanism. An elastic member 48 is laid in a compressed state between the coupling member 10 and the top link 4 and elastic force acts in the direction to enlarge the angle between both the coupling member 10 and the top link 4. Consequently, force to drop the lower link 6 works, a working machine 11 is grounded, then the coupling member 10 can be rapidly dropped so that the coupling member 10 can be readily released from the working machine 11.