PURPOSE: To manufacture a thin pipe with good working efficiency by arranging plural molds having thin pipe-like cavity part branched from a runner into a reducing pressure vessel, making the pressure in the reducing pressure vessel lower than the pressure in the molten metal and pouring the molten metal into the mold.

CONSTITUTION: The mold 1 is formed with a cavity as the runner and the complex mold 3 with plural molds 1 branched radically arborescently around the circumference of a main runner 3a having opening in vertical direction of a support is formed with lost wax method. This complex mold 3 is contained in the reducing pressure vessel 5 and at the time of exposing an inner part of the reducing pressure vessel 5 to the prescribed negative pressure atmosphere by evacuating with a vacuum device, the molten metal in a metal melting furnace 7 is sucked from the lower end part 3b and flows into mold 1 from sprue respectively through the main runner 3a. At the time when the pouring of the molten metal completes, operation of the vacuum device is stopped and then, the molten metal packed into the main runner 3a is caused to flow down in the metal melting furnace 7 by gravity. After the molten metal in each mold 1 solidifies, the complex mold 3 is taken out and by breaking the mold 1, the casting product is obtd.