  Using H I absorption alone, we attempt to separate H I absorption lines in
quasar spectra into two categories; HDLs (Higher Density Lines) and LDLs (Lower
Density Lines), and we discuss the difference in their physical properties. We
deblend and fit all H I lines with Voigt profiles, and make an unbiased sample
of H I lines covering a wide column density range (12 < log N_HI < 19 cm^-2).
To reduce the influence of line blending, we simultaneously fit several Lyman
series lines. As a result of a two-point correlation analysis, we found that
higher column density H I lines are clustering at $\Delta$ v < 200 km/s, while
lower ones at $\Delta$ v < 100 km/s. We define HDLs as H I lines with 15 < log
N_HI < 19 cm^-2 and all H I lines within +/- 200 km/s of a line with log N_HI >
15 cm^-2, and LDLs as others with 12 < log N_HI < 15 cm^-2. We found that the
HDLs have smaller minimum b-values for a given column density than the LDLs.
This difference is successfully reproduced by our Hydrodynamic simulation. The
LDLs seem to be cool or shock-heated diffuse IGM gas, while the HDLs are likely
to be cooler dense gas near to galaxies.
