To provide a non-locking preventive device notifying a resident of the fact that the lock is in a non-locking state, on the spot of unlocking.

The non-locking preventive device X added to the lock Y has a notification control mechanism 21 mechanically interlocked with a thumb-turn knob and starting and stopping a notifying means 16 through a switch 17. The notification control mechanism 21 comprises a control cam 22 engaged and interlocked with the thumb-turn knob, and a control lever 23 controlled by the control cam 22, and starts the notifying means 16 by the control lever 23 through the switch 17 when the thumb-turn knob is in a release position. The non-locking preventive device X further comprises a notification stop means 25 slid in a predetermined direction by operating force to push the control lever 23 back into a locking position.