Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: III.
  The Total Neutral Gas Disk
  We present newly obtained three-dimensional gaseous maps of the Milky Way
Galaxy; HI, H$_2$ and total-gas (HI plus H$_2$) maps, which were derived from
the HI and $^{12}$CO($J=1$--0) survey data and rotation curves based on the
kinematic distance. The HI and H$_2$ face-on maps show that the HI disk is
extended to the radius of 15--20 kpc and its outskirt is asymmetric to the
Galactic center, while most of the H$_2$ gas is distributed inside the solar
circle. The total gas mass within radius 30 kpc amounts to $8.0\times 10^9$
M$_\odot$, 89\% and 11\% of which are HI and H$_2$, {respectively}. The
vertical slices show that the outer HI disk is strongly warped and the inner HI
and H$_2$ disks are corrugated. The total gas map is advantageous to trace
spiral structure from the inner to outer disk. Spiral structures such as the
Norma-Cygnus, the Perseus, the Sagittarius-Carina, the Scutum-Crux, and the
Orion arms are more clearly traced in the total gas map than ever. All the
spiral arms are well explained with logarithmic spiral arms with pitch angle of
$11\degree$ -- $15\degree$. The molecular fraction to the total gas is high
near the Galactic center and decreases with the Galactocentric distance. The
molecular fraction also locally enhanced at the spiral arms compared with the
inter-arm regions.
