PURPOSE:To reduce equipping cost of casting apparatus by forming a sand molding part having lost foam pattern in a casting flask having single structure, arranging an annular gas venting duct at outer periphery in the casting flask and communicating with gas exhaust pipes in the casting flask. CONSTITUTION:The plural gas exhaust pipes 6 are arranged at inside of the front frame body 2 in the casting flask 1 and gas exhaust holes 7 are arranged as zigzag. The upper part of the gas exhaust pipe 6 penetrates the frame body 2 and communicates with gas venting duct 12 formed at outer periphery thereof. The lost foam pattern 28 is arranged as connecting with runner part on the bed sand 29 in the base seat part 3, and the frame body 2 is fixed as connected with a joint hook 5. Sand molding part 1 is formed with molding sand 8, and molten metal is poured from a sprue 33 under reduced pressure. The air sucked in the gas exhaust pipes 6 is exhausted through the gas venting duct 12 and the casting is formed by lost-foaming of the pattern. As the pressure in the sand molding part 31 is reduced with the casting flask 1 having single structure, the equipping cost of the apparatus is reduced.