PURPOSE: To improve flexibility in two directions on the bent front surface by forming strips by connecting identical trapezium segments so that two edges may have angles, and connecting mutually adjacent strips in the mirror image relationship into a corrugated sheet having alternative projecting and recessed parts.

CONSTITUTION: Identical trapezium segments 3 having no parallel edge are connected to each other so that two mutually adjacent edges 5 may have angles. Strips 9 are arranged and joined so that the mutually adjacent strips 9 may be mirror images of each other, and a corrugated sheet in which projecting parts and recessed parts alternately exist is formed. Accordingly, flexibility is provided in the length and width directions, and the sheet can be simultaneously stretched in both directions.