To provide a muffler functioning when unlocking a dead bolt in complete locking.

A lock includes a dead bolt which is provided with a rack gear along the moving direction of the dead bolt. On the other hand, the dead bolt has a pinion gear engaging with the rack gear, and a rotation control damper member of which brake force is different depending on the rotational direction is disposed in a lock box. The rotation control damper member is composed of a case fixed in the lock box, the pinion gear fixed to a projecting tip part of a torque axis of a rotor installed in the case and engaging with the rack gear, a control projection or rotation control blade having a brake surface provided on the rotor, and a highly viscous material filled in the case. Viscosity resistance of the highly viscous material against the control projection or rotation control blade having the brake surface is higher when the dead bolt is brought into an unlocking state from a locking state based on the operation force of a duplicate key than when the dead bolt is brought into the locking state from the unlocking state.