PROBLEM TO BE SOLVED: To improve operability of opening operation for bringing an opening/closing connection annular part (opening/closing arm) into an opened annular state, the opening/closing connection annular part retained in a closed annular state by means of engagement with an opening motion prevention part retaining an engaged posture by a strong energization force.SOLUTION: The connection tool is composed of: a case body 1; an opening/closing connection annular part 2 including one end engageable with the case body 1 and freely rotated to be opened/closed; an opening motion prevention part 3 preventing an opening motion of the opening/closing connection annular part 2; and a cancellation lever part 4 canceling an opening motion disabled state of the opening/closing connection annular part 2. The cancellation lever part 4 presses and moves the opening motion prevention part 3 by double force action through the lever principle, the opening motion prevention part retaining an engaged posture with the ope
