To provide an easily operable coming off preventive device.

A main body 3 is provided with a coming off preventive member 14 and a pinion 13 and rack 8 which moves the coming off preventive member 14, and the rack 8 is moved by a spring 12 in one direction to a coming off preventive position. The rack 8 is provided with an operative projection 10 and a stopper 11, and the stopper 11 can be locked to and uncouplable from the end of an oblong hole 5 in the main body 3. When the rack 8 is moved by the operative projection 10 in the other direction against the spring 12, the coming off preventive member 14 comes to a non-coming off preventive position and the stopper 11 locks to the end of the oblong hole 5. The stopper 11 is uncoupled from the end of the oblong hole 5 by pressing the operative projection 10.