To provide an internal lock with superior crime prevention property by making it difficult for an invader to unlock a lock operation member.

This internal lock comprises a casing 2 mountable on the indoor side surface of a door, a lock member 3 installed in the casing 2 so as to be projected from the casing 2 to a door frame, the lock operation member 4 installed on the outside of the casing 2 so as to be operated by a user, and an interlocking mechanism 5 installed in the casing to change the attitude of the lock member 3 to the lock position or the unlock position according to the operation of the operation member 4 by the user. The internal lock further comprises a lock holding means 6 controlling the projecting and recessing motion of the lock member 3 normally and allowing the projecting and recessing motion of the lock member 3 only when it is unlocked by the user.