Even if the coupling surface of an arm side attachment and a tool side attachment is directed in a direction other than the horizontal direction, the tool side attachment can be automatically separated and detached from the arm side attachment. When exchanging a tool, the worker does not have to forcibly pull out and detach a tool side attachment from an arm side attachment, a tool can be exchanged automatically, and the exchange work can be performed easily in a short period of time. When the cam member 17 is shifted from the unlocking position to the locking position, locking balls 13 which slide on locking inclined tapered surfaces 17a to be shifted to the outside along the radius direction are engaged with engaging inclined tapered surface 23a so as to mutually couple an arm side attachment 3 and a tool side attachment 5. When the cam member 17 is shifted from the locking position to the unlocking position, the engagement of the locking balls 13 with respect to the engaging inclined tapered surfaces 23a is released, and unlocking balls 15 which slide on thrusting inclined tapered surfaces 17b to be shifted to the outside along the radius direction slide on the separating inclined tapered surfaces 23b to enable to thrust the tool side attachment 5 from the arm side attachment 3.