such that the safety door on the side that is not being operated does not continue to move under inertia and therefore safety can be achieved it should be noted that although in the embodiment described above the hinge member 11 d is provided on the engaging means 11 and the engaging portion 19 is provided on the primary safety door 10 a alternatively these may be reversed with the hinge member 11 d provided on the primary safety door 10 a and the engaging portion 19 provided on the engaging means 11 fig5 a and 5 b illustrate another aspect of the engaging means of the embodiment that is an example of the reversal described above a swingable member 21 similar to the hinge member 11 d described above is provided on the primary safety door 10 a with a projection 22 that engages the swingable member 21 provided on the engaging means 11 then the swingable member 21 is urged to the state shown in fig5 a either under its own weight or by urging means such as a spring although the swingable member 21 can rotate about an axis 21 a in a direction indicated by an arrow in fig5 a it cannot rotate in the reverse direction as a result as shown in fig5 b when the primary safety door 10 a and the secondary safety door 10 b are moved and the swingable member 21 moves from right to left in the drawing with respect to the projection 22 the swingable member 21 rotates and.