PURPOSE: To heighten strength and, at the same time, to reduce costs for production by constructing a rectangular panel of two parts that are split at the centerlines of a recessed groove and a projecting stripe provided respectively to each one of long sides of the panel, and by forming each one of the parts of a skin layer, a sticker layer and an expanded resin core layer.

CONSTITUTION: A rectangular panel 10 for building is formed with a recessed groove 1, 1 provided to the center of the width on one long side thereof and a projecting stripe 2, 2 provided to the center of the width on the other long side thereof, so that the panels become capable of being joined by tongue-and-groove joint. The panel 10 is made of two parts that are split at the centerlines of the recessed groove 1, 1 and the projecting stripe 2, 2, and each one of the above-mentioned two parts is formed of a skin layer 3 made of a steel plate or the like, a sticker layer 4 made of paper or nonwoven fabric being formed on the surface facing the surface of the other part of the panel, and an expanded resin core layer 5 incorporated in the skin layer 3. Then the two parts each of which is composed of the skin layer 3, the sticker layer 4 and the resin core layers 5, are united into one piece with the surfaces of the sticker layers 4 put together. Thereby a panel having long sides can be manufactured easily, and desired panels 10 can be obtained only by cutting the long sides of the above-mentioned panel in a specified length.