PURPOSE: To enhance the stability in the contracting ratio of a multi-purpose panel and reduce the warp and distortion by forming partitioning reinforcement ribs with a smaller wall thickness than the framing of the panel body, and also forming it at the same level as the framing.

CONSTITUTION: A panel body 2 is embodied in the form of a square frame whose under-side is left open, and a framing 6 is raised at the peripheral edge of the rear surface 5 of this panel body 2. By the compression molding, partitioning reinforcement ribs 4 in the form of honeycomb core are formed which are raised directly at the rear surface 5 and the inside circumferential wall surface 7. These ribs 4 are given a wall thickness smaller than the framing 6 of the panel body 2 and are made with the same level as the framing 6. This stabilizes the contracting ratio and allows reducing the warp and distortion.