To make easy bending and increase rigidity of a mold by forming a resin skeleton buried in a window mold in a variable cross-sectional shape.

An upper mold part 1A including a corner mold part 1C is formed in a uniform cross-sectional shape. Also a side mold part 1B connected continuously to the upper mold part 1A is formed so that its cross-sectional shape is increased gradually toward the direction opposite to the upper mold part 1A. Then a resin L-section skeleton material 15 is buried only in the side mold part 1B. In addition, the skeleton material 15 is formed so that its cross- sectional shape is increased gradually toward the direction opposite to the upper mold part 1A according to a variation in the cross-sectional shape of the side mold part 1B.