To provide a key-hole cover for burglarproofness capable of preventing a picking damage, utilizing a conventional lock by mounting an accessory for preventing a picking to the lock for a door, a key with a key case and a method for installing the key-hole cover.

The lock is constituted such that the key-hole cover 1 covers a key hole 4 for the lock and fixed by a U-shaped hold-down metal fitting 10, the key 5 is housed in the key case 6 prepared in conformity with the key hole 3 for a part 2 for the key hole, the key 5 is projected from the key case 6, the front end of the key case 6 and the key 5 are inserted into the key hole 3 for the part for the key hole and the lock is unlocked and locked.