To provide a lock mechanism of a slide adjuster for a vehicle capable of saving a space for installation and having high strength.

A lock member fitting hole 4f is bored in an upper wall part 4a of an upper rail 4, and a lock member 42 is fitted in the lock member fitting hole 4f, and while the lock member 42 is fitted to the upper rail 4 freely to swing around a support shaft 40 fitted to a part of the upper rail 4. The upper rail 4 is locked by the lock member 42 in relation to a lower rail 2, and lock is released by swinging the lock member 42.