[en] Eigen-vortical-axis line proposed as a vortical axis is analysed with respect to the characteristics of the passage of the core region of a vortex in isotropic homogeneous turbulence. Comparison with the vorticity line shows that the difference between them is derived from the geometrical features of a vortex especially vorticity components parallel to swirl plane in weak region of a vortex. Eigen-vortical-axis line tends to pass through the core region with strong intensity of swirling (swirlity) and vorticity, and high flow symmetry. (paper)