PURPOSE: To obtain a building plate free from wrinkles and resin leakage by a method wherein both sides of a sheet coated with ultraviolet-setting coating material is folded upward to from a nearly U-shaped section and after being cured by means of irradiation of ultraviolet rays, expandable resin is injected, expanded and cured.

CONSTITUTION: A soft and thin sheet 2 such as printing paper, plastic sheet or aluminum foil is drawn continuously out of a wheel and coated uniformly with ultraviolet-setting coating material through a UV coater 4 all over the sheet. The sheet 2 is formed by a roll 5 in a U-shape and irradiated by lutraviolet rays in a UV irradiating furnace 6 and cured. Urethane resin is injected into said shaped sheet from a head 7 and expanded and cured during the passing through the double conveyor 8 to obtain a plate used for building.