PURPOSE: To prevent surely a key from being allowed to remain with simple constitution by providing a magnetism sensitive switch closed when a magnet key is inserted in a key cylinder lock of a vehicle to give an alarm on the basis of the output of a door switch when the key is allowed to remain inserted.

CONSTITUTION: When a magnet key 7 having a key body 7a provided on the upper portion with a plurality of magnets g1, g2 is inserted in a key hole 8 of a key cylinder lock 1, a plurality of magnet pin tumblers 9a, 9b accommodated in a bore of an outer tube member 2 and a groove of an inner tube member 5 of the key cylinder lock 1 spring up against a spring 10 with a magnetic repulsion force to close a magnetism sensitive switch 11. While the magnetism sensitive switch 11 is closed, when a door of a vehicle is opened, a door switch 12 is turned on and an alarm 14 such as a buzzer connected in series to both switches 11, 12 is blown to alarm that the magnet key 7 is being inserted into the key cylinder lock 1.