A plurality of connecting members are provided to a tool adaptor to which a tool is mounted, a plurality of connecting jaw members having inclined faces which are engaged with and detached from the connecting members when the connecting jaw members are displaced in radial directions, a rod that expands and contracts due to operation of air pressure, and a cam-type converting mechanism for converting the expanding/contracting operation of the rod to operations of displacement of the jaw members are provided to a mounting unit mounted to a robot hand, and a stopper portion for preventing the connecting member from being detached and falling from the inclined face when the air pressure reduces is provided to each the jaw member.