PURPOSE: To discharge the gas generated during casting and to prevent generation of the blowholes in the upper part of the hollow passage of a piston having the hollow passage in the upper part in the stage of casting said piston by providing hollow holes communicating with a vacuum pump to the center core and side cores of a metallic mold core.

CONSTITUTION: A molten metal is poured through a pouring port 14 into the metallic mold formed of a cope 11, a master mold 12, a guide plate 10 of the mold, a center core 3 and side cores 4, 5. A core center 2 is surely fixed into the four recesses 4c provided in the upper part of the side cores and is correctly held in the normal position. A piston 1 is then formed. A vacuum pump 9 is run prior to pouring of the molten metal to discharge positively the gas accumulating in the hollow passage 1a of the piston through the hollow holes 3a, 4a, 5a of the center core 3 and the side cores 4, 5 and further the disc body 2, leg part 2a, core print 2b and vent plug 13 of the core center having good air permeability.