To form the protruded curved surface of a groove into a smooth surface same in shape.

In the method for forming the groove to a building panel by forming a V-shaped groove 7 to the surface 5 of the building panel and pressing the roll surfaces 28e, 30e and 32e of press rolls to the opening edge 10 of the V-shaped groove 7 to form the protruded curved surface 11 to the V-shaped groove 7 on the opening side thereof, the cross sections of the roll surfaces 28e, 30e and 32e formed by cutting the press rolls by the surfaces containing the center lines 28b, 30b and 32b of the rotary axes of the press rolls are formed into an outward protruded V-shape and the protruded curved surfaces 11 are formed by pressing pressure rolls to the building panel in a multistage fashion in the order from the press rolls small in vertical angles 28f, 30f and 32f of the roll surfaces to the press rolls large in vertical angles.