To provide an inexpensive falling prevention screw improved in a sure coming-off prevention action and recyclability.

In this falling prevention screw, a fastening thread portion 11 is formed on a distal end side of a leg portion 10, and a locking thread portion 12 which is opposite to the fastening thread portion 11 and has a larger thread outer peripheral locus circle than that in the fastening thread portion 11 is formed on a head portion side. A member to be fastened is provided with a coming-off prevention hole 32 having a larger diameter than that in the fastening thread portion 11 and a smaller diameter than that in the locking thread portion 12. When the falling prevention screw is mounted in the member to be fastened, the fastening thread portion 11 is penetrated into the coming-off prevention hole 32, the locking thread portion 12 is rotated in a direction opposite to a screwing direction of the fastening thread portion 11, and the coming-off prevention hole 32 is locked between the head portion 2 and the locking thread portion 12. Even if the fastening thread portion 11 comes off from a prepared hole of a workpiece due to looseness, the screw does not come off from the coming-off prevention hole to surely prevent from falling. The screw is made of a single material to improve recyclability.