We derive radial and vertical distributions of HI and H _ { 2 } 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 ^ { \circ } \sim 64 ^ { \circ } and galactic latitude b = -2 ^ { \circ } \sim + 2 ^ { \circ } .
The thickness of the molecular dominant region ( f _ { mol } \geq 0.8 ) is approximately constant ( 109 \pm 12 pc ) at galactocentric distance R \simeq 4.7 - 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 H _ { 2 } gases in the interstellar matter . Key Words : Atomic Hydrogen — the Galaxy — Interstellar Matter — Molecular Hydrogen