To provide a lock mechanism for a door in which the inner space of a lock box is effectively utilized by reducing the number of parts and a damping lever functions quickly.

In this lock mechanism for a door, the front end of a lock lever pivotally supported in the lock box locks a latch member when locking. The center part of a pendulum stopper is pivotally supported on a second pivot shaft. The pendulum stopper is configured in such a manner that the top end having a weight portion is brought into contact with a support member while being always biased so as to be kept in a predetermined attitude by the spring force of a pendulum spring wrapped around a second pivot shaft. The bottom end of the pendulum stopper faces the gap between a horizontal contact portion provided to the front end of the lock lever 5 and a first pivot shaft. When the predetermined attitude of the pendulum stopper is collapsed by a torque during the excitation, the bottom end quickly blocks the contact portion, and stops the rotation of the lock lever in the unlocking direction.