PURPOSE: To sharply shorten a cooling period and to improve the productivity of a molding, by applying a mold having a concave surface to a block molding deivice for molding a bolck made of a bead method foaming polystyrol.

CONSTITUTION: In molding a bead method foaming polystyrol block, a mold, whose surface is concave, is applied to a block molding device. This permits the shortening of a cooling. When a value obtained resulting from the reduction of swelling value allowable to a product from a swelling value of the product being desired to that extent under a demolding pressure is applied to a concave surface of the mold, the aim can be achieved successfully. It is desirable to support by means of projection whose height may correspond to a concave surface maintaining the formation of a concave surface in the mold surface.