PURPOSE: To enable the automatic locking with the simple structure by utilizing an existing latch assembly.

CONSTITUTION: A latch assembly 2 is fixed to a lock box 1. In the lock box 1, a retractor 4 to be slid for guide and energized forward is engaged with the rear end of a latch 21. The rear end of the latch 21 is allowed to be relatively moved backward in relation to the retractor 4 at the engagement part with the retractor 4 under the latch projecting condition. A rotary shaft 5 to be turned by an operating member 51 is engaged with the retractor 4 through an engaging member 53. One end of a stopper piece 7 supported inside of the lock box 1 freely to be turned is coupled with an operating rod 61 of a solenoid 6, and in response to the displacement position of the operating rod 61, the other end of the stopper piece 7 is selectively moved into the operating area of the retractor 4 to restrict the retractor 4 at the forward position.