PURPOSE: To inexpensively prepare a light wt. sheathing board having a reinforcing plate therein and excellent in rigidity, by performing the blow molding of synthetic resin by using a specific mold provided with a slide plate.

CONSTITUTION: A parison 17 is extruded into split molds 11a, 11b and, after clamping, a vertical slide plate 14 and a horizontal slide plate 15 provided in disired places are extruded into the molds to partially grasp and press both side walls of the parison 17 to form welded parts 18, 19. Subsequently, when the slide plates 14, 15 are returned to a position where the leading ends thereof are brought to the same surface as the mold surfaces and, after fixed at said position, air is blown into the parison 17, the parison 17 is expanded to fully fill the mold cavity and formed into a main body consisting of a left surface plate 2, a right surface plate 3 and side walls 4 while the above mentioned welded parts 18, 19 are drawn into plate shapes to be formed into a vertical reinforcing plate 5 and a horizontal reinforcing plate 6 and a sheathing board 1 having the reinforcing plates arranged in the desired places thereof is obtained.