A separation preventing device that prevents separation of a fluid pipe in a pipe axis direction, includes a main body externally fitted to an outer peripheral surface of the fluid pipe and has a recess portion facing the outer peripheral surface; a locking member housed in the recess portion in a tiltable manner and is able to bite into the outer peripheral surface of the fluid pipe; a pressing member provided in the main body member and presses the locking member so as to bite into the outer peripheral surface of the fluid pipe; and an interposition member interposed between the locking member and the pressing member, wherein the interposition member is equipped with a tapered surface coming into contact with an outer surface of the locking member, and is housed in the recess portion in a state of having an acceptable movement value of a predetermined width in the pipe axis direction of the fluid pipe.