Abstract: A buckle for seat belts which comprises a buckle body and a tongue plate engageable with the buckle body when inserted thereinto through an opening formed in the front end of the buckle body, the buckle body being provided with: (a) a frame comprising a bottom plate above which a path for insertion and ejection of the tongue plate is defined and a pair of side plates, (b) an ejector being mounted slidably on the bottom plate and urged always toward the opening, (c) a hook plate being supported on the side plates of the frame and having a lock pawl, (d) a pair of control slits being formed in the side plates of the frame, each control slit forming a narrow portion in the front part and an upwardly enlarged portion in the rear part, (e) a release bar having both ends disposed in the control slits, and (f) a manual operation assembly for buckle for buckle release, characterized in that (i) the release bar is shaped in a flat rectangular form in cross section, (ii) a supporter therefor is positioned on the hook p