To prevent forgetting to lock, by fitting a preventive device against forgetting to lock, provided with a loudspeaker, an electric circuit board, a mercury switch, and a battery, to a lug of a door lock and switching on at the unlocking time to sound a music.

A loudspeaker 3, a mercury switch 4, an electric circuit board 5 to sound musics, and a battery 6 are fixed to a body 9 having fitting holes for attaching on a lock, in order to form a preventive device against forgetting to lock. The fitting holes of the body 9 are fitted to the lug of a door lock and stuck with a both face-adhesive tape 10 so that the fitting holes become horizontal against the ground face. In this time. the mercury switch 4 is put off in the state the fitting holes are horizontal and also it is set to put on when it becomes vertical. When the lug of lock is turned 90 degrees on opening the door, the body is also turned 90 degrees and the mercury switch 4 is put on and the circuit board 5 sounds a music to inform that the door is opened. And hence, it prevents forgetting to lock.