  We derive radial and vertical distributions of HI and H2 gas densities in our
Galaxy by using the terminal velocity method. We calculate the molecular
fraction (f_mol) defined as the ratio of the molecular hydrogen to total
hydrogen gas density at galactic longitude l= 33 to 64 deg and galactic
latitude b=-2 to +2 deg. The thickness of the molecular dominant region f_mol >
0.8 is approximately constant (109 +/- 12 pc) at galactocentric distance R =
4.7 to 7.2 kpc. The molecular fraction decreases suddenly at a critical height
from the galactic plane, below which the gas disk is almost totally molecular,
while it is almost atomic beyond this height. We show that the vertical f_mol
variation can be reproduced by a model which takes into account the phase
transition between HI and H2 gases in the interstellar matter. Key words:
Atomic Hydrogen --- the Galaxy --- Interstellar Matter --- Molecular Hydrogen
