PURPOSE: To produce a product having no gas defect and stable quality by beforehand forming a hollow part for gas venting at the time of molding a master mold in vertical type continuous casting and forcedly sucking gas from the hollow part after pouring molten metal.

CONSTITUTION: At the time of molding the master mold 2, the hollow part 4 is formed as extending to the tip side of a supporting core print 3a of a core 3 and after that, the core 3 is combined. Then, at position as apart from master mold molding means 1 in the prescribed distance along a conveying line, a communicating hole boring means 5, molten metal pouring means 6 and gas suction means 7 are set in series in this order. The communicating hole boring means 5 bores toward the hollow part 4 to form the communicating hole 8. The master mold 2 forming the communicating hole 8 is shifted in step by step and stopped at just below the molten metal pouring means 6 to execute automatic molten metal pouring. After pouring the molten metal, the gas is generated while the molten metal in the master mold 2 solidifies and stored into the hollow part 4 from the mold. Successively, the gas suction means 7 forcedly sucks the gas in the hollow part 4 through the communicating hole 8 and discharges it outside.