PURPOSE: To reduce the cost of material and weight of a web core and make it difficult to deform according to the lapse of time by using the web core which is free to bend and deform.

CONSTITUTION: A pair of vertical blocks 25, an upper block 27, and a lower block 26 are positioned at four sides of a web core to form a core assembly 2. Also surface plates 4 and 5 in which the surface faced on the core assembly side is coated with adhesive agent are positioned on both upper and lower sides of the core assembly 2 to put the surface plates on the core assembly. Then the surface plates and core assembly are heated, pressed, and formed by holding them between the curved projecting surface of a male-formed die 6 and the curved projecting surface of a female-forming die 7 to form the surface plates and core assembly in curved shape.