To provide a picking prevention cover which requires no work and tool and can be easily installed to a door by anyone; a key can be moved to opening and closing position by a single hand and its removal is not required; when a strong force is applied, an alarm is activated thereby informing an abnormal situation; and also this cover can be used as a one door/two lock.

A hook (2) and a U-shaped groove (3) are provided for a guard (1), a plate (5) having a magnet (4) and a rubber (6) are fixed, a plurality of spring U-shaped grooves (8) and spring hooks (9) are attached externally, a spring plate (10) is fixed, a key (13) having a cover (12) and fittings (14) are provided for a case (11), a plurality of grooves (16) are provided at an opening portion (15) and an inner side, and a sensor (17) with an alarm is built in as the feature of the cover.