To provide a device for preventing forgetting of locking, which is simple in structure, dispenses with construction work to a door frame, and can surely prevent forgetting of locking, and to provide a method of installing the device.

The device 1 for preventing forgetting of locking comprises a permanent magnet 2 mounted to a door frame T, and a device body 3 mounted to a door D. The device body 3 stores: a reed switch 31 for detecting magnetism from the permanent magnet 2; a microswitch 32 for detecting a locking state of a thumb-turn S of the door D; and a buzzer 34 for inviting attention of a resident to the forgetting of locking, in a box-shaped case. Since the reed switch 31, the microswitch 32, and a battery 33 are connected in series, the buzzer 34 sounds when the reed switch 31 detects the magnetism of the permanent magnet 2 and the microswitch 32 detects an unlocking state of the thumb-turn S.