In a press for forming a long and narrow plate or strip of metal or synthetic resin into an elongated beam having an L-shaped section, of the type having a lower fixed frame provided with a female die on the top thereof and a hydraulically operated upper movable frame provided with a male die on the bottom thereof, a long space adjacent and parallel to the lower surface of the movable frame is provided in the movable frame. In the space are provided a plurality of small hydraulic deflection cylinders with pressure-fluid paths connected to the pressure-fluid paths of the main hydraulic cylinders to drive the movable frame. The space provided with a plurality of the small hydraulic deflection cylinders is so located that the center portion of the lower part of the movable frame is deflected counter to the deflection of the movable frame under press-forming load. When hydraulic pressure fluid is supplied to the main hydraulic cylinders to drive the movable frame for forming the plate by means of the dies fitted on the frames, the pressure fluid is also supplied to the plurality of the small hydraulic deflection cylinders in the space of the movable frame and the primary deflection of the frames under load is compensated for, so that a beam of L-shaped section free from convex or concave warp at its center portion is produced.