PURPOSE: To produce easily a piston provided with a combustion chamber having an inside circumferential surface with thread grooves of high accuracy by disposing a core formed with thread grooves on the outside circumferential surface in a metallic mold, charging a molten metal in the mold and unscrewing the core from the cast piston.

CONSTITUTION: A core 1 formed with thread grooves of a specified pitch on the outside circumferential surface is disposed in the cavity 4 formed by stationary and moving molds 2, 3, and is fixed by means of a hook 9 via a central shaft 103 of the core. The top surface 102 of said core is fitted into a fitting hole 5 having a projecting rate (t) in the upper part of the molds 2, 3. On the other hand, a core 8 for a piston skirt is disposed in the lower part in the cavity 4 and is fixed by means of a setting means 11. An Al alloy as a molten metal is charged in this state into the mold through a sprue 12, and is cooled to solidify. The core 1 is pulled in a feed direction H from the casting removed from the molds after parting of the molds while said core is rotated around the central line (l). A piston 6 formed with a combustion chamber having the prescribed thread grooves on the inside circumferential surface of a top surface 601 is thus obtd.