Orbital elements: Difference between longitude of perihelion and argument of perifocus

I know the answer now. Turns out that longitude of perihelion ($overlineomega$) is defined as the sum of longitude of ascending node ($Omega$) on the ecliptic plane and argument of perifocus ($omega$) on the orbital plane: ($Omega$ + $omega$). It's strange to add up two non-coplanar angles, even when the inclination between the two planes is very small, but that's what it is.