To prevent a lockout situation when closing an interior door.

In this lockout preventing device for a thumb-turn lock of the interior door, an operating rod is disposed in a horizontally retreating manner in a lock case behind a latch, and a release constituent member for a thumb-turn tumbler having a lever engaged with the operating rod is built in. When the latch abuts on the edge of a door frame and retreats in closing the interior door, the release constituent member separates a driving arm of the thumb-turn tumbler from a locking piece in cooperation with the retreating movement of the operating rod. The locking piece for a latch cam thereby moves in a direction of releasing engagement by the spring force of a return spring.