To provide a double lock for a door easily responding to the safety control of a building by easily attaching a picking prevention lock to an existing key of the building door at a low cost using a device easy of a laymen.

This double lock covers a lid openable/closable a keyhole of a first lock of the door and opening the lid by a second lock. This double lock is formed into an integral structure by bolting the physically strong first key fixed to the door, the lid having the picking prevention second lock, and a box frame connected to the lid by a hinge and attached to the lid covering the first lock so as to fix the double lock to the door of the building. The picking prevention second lock comprises a core body positioned in a lid casing and having locking magnetic pins in multiple recessed grooves, and a pair of plates capable inserting the magnetic card key having magnetism causing an attracting action and a repulsing action to/from the locking pins for providing an unlocking function, a locking retaining function, and a locking function.