PURPOSE: To compress the molding sand charged to the periphery of a pattern in a molding flask by a diaphragm and to form a casting mold by supplying pressure air into a pressure vessel body provided with the diaphragm having a fluid-tightly installed bottom surface.

CONSTITUTION: Nuts 6a, 6b are removed and pressing parts 7W13 are removed. A pattern 3 is placed on an upper molding base 4a, and after molding sand 1 is charged, the parts 7W13 are placed on the flask 2 and both ends of a supporting arm 12 are tightened and fixed by means of the nuts 6a, 6b. Pressure air is then introduced through an air connecting pipe 11 into a pressure vessel 8. As a result, a diaphragm 7 is inflated, by which the molding sand 1 in the upper part is compressed downward and the top end faces of a retaining plate 21 and the flask 2 are depressed downward. The lower part of the molding sand 1 and the molding sand in the peripheral part of the pattern 3 are thus thoroughly compressed.