To provide a burglarproof key, by which an aluminum sash is not opened even when a crescent lock turned on from the inside is unlocked by damaging a part of a large glass fitted to a frame for the aluminum sash and which has a high safety and the cost of which is reduced.

The key is constituted so that a cylindrical rod 1 and a yoke 2 are joined and a device formed of a disk type permanent magnet 3 introduced in the yoke is inserted to the inside 5 of a vessel formed of vinyl chloride and the lock is locked and unlocked by inserting or extracting the rod 1 into or from a hole 23 formed to the aluminum sash by a repulsion in the case of the same magnetic pole (an pole N and the pole N) between two disk type permanent magnets when the disk type permanent magnet 10 on the outside is brought near and an attraction in the case of the reverse magnetic pole (the pole N and the pole S).