To solve the problem that most of usually devised devices for preventing forgetting to lock are used to be attached to a key body and to confirm locking in a going-out place, not established when plural persons use one device, not attachable to the key body due to the shape of a key or the key manufacturer, issuing an alarm upon locking, causing a key body to be manufactured, or require perforation for wiring, therefore such devices cannot always be easily attached by anyone and not always be inexpensively manufactured and provided at a low price.

Because the alarm sound of an inventive device does not stop as far as locking is not performed, forgetting to lock is surely prevented for one who is unconsciously going out thinking something and even for forgetful persons and elderly persons. A portion which senses locking is simply made of an insulation plate and a contact metal fitting so that it has excellent durability at a low cost and does not limit manufacturers and key shapes. Furthermore, the device is only adhered easily so that it can be simply attached by anyone.