PURPOSE: To prevent the deformation of an annular edge core and to uniformize the thickness around the annular space in a piston by arranging solidification promoting parts to non-supporting part which is easily expansion-deformed by receiving thermal influence of molten metal, in the annular edge core for forming the annular space in the top part of the piston.

CONSTITUTION: At the time of casting a raw piston, the annular edge core 6 for forming the annular space in the die is supported and cast to manufacture the piston for engine. Then, in the annular edge core 6, plural solidification promoting parts (molten metal invading holes) 6b are arranged to the non- supporting positions, which are apart from supporting positions 6a (supporting pin inserting holes) supported with the die and are easily expansion-deformed by receiving the thermal influence of the molten metal, are arranged. In this constitution, at the time of executing the casting by pouring the molten metal into a cavity in the die, the annular edge core 6 is heated and intends to expansion-deform toward outer part in the radial direction, but by restricting with the solidified metal in the invading holes 6b, the thermal deformation is restrained to form the annular space having almost the complete roundness.