A partition plate (cast-in object plate member) is preliminarily set in an intake-port forming sand core, by which an intake port of a cylinder head is molded, and cast in during cast molding of the cylinder head for partitioning the intake port. The partition plate has both distal ends, spaced apart in a longitudinal direction, one of which corresponds to an intake-side distal end at which an acute-angled end face is formed to induce core breaking in a port core at an area closer to the intake-side distal end.