A spark plug for an internal combustion engine and related manufacturing method are disclosed having a metal shell, a porcelain insulator, a center electrode and a ground electrode. The center electrode includes a substantially columnar shaped base material body, and a substantially square shaped rod-like Ir alloy tip bonded to a leading end portion of the base material body. The leading end portion has a diameter D2 smaller than a diameter D1 of the base material body. The Ir alloy tip has a square shape with a long diagonal line supposed to form a diameter A of a circumscribed circle CA of the Ir alloy tip whose inscribed circle CB is coaxial with the circumscribed circle CA and has a diameter B, with four diameters A, B, D1 and D2 lay in the relationship expressed as D1>A>D2>B.