  By mapping the molecular fraction of the Galactic Center (GC), we
quantitatively address the question of how much molecular and central the CMZ
(Central Molecular Zone) is. For this purpose we analyze the CO and HI-line
archival data, and determine the column- (surface-) and volume-molecular
fractions, $f_{\rm mol}^\Sigma$ and $f_{\rm mol}^\rho$, which are the ratio of
column-mass density of H$_2$ projected on the sky to that of total gas (H$_2$ +
HI) from the line intensities, and the ratio of volume-mass densities of \Htwo\
to total gas from the brightness temperature, respectively. It is shown that
$f_{\rm mol}^\Sigma$ is as high as $ \sim 0.9-0.95$ in the CMZ, and $f_{\rm
mol}^\rho$ is $0.93-0.98$ in the GC Arms I and II attaining the highest value
of $\sim 0.98$ toward Sgr B2. The expanding molecular ring (EMR, or the
parallelogram) has a slightly smaller $f_{\rm mol}^\Sigma$ as $\sim 0.9-0.93$.
We define the CMZ as the region with $f_{\rm mol}^\Sigma \ge 0.8-0.9$ between
the shoulders of plateau-like distribution of H$_2$ column density from
$l=-1^\circ.1$ to $+1^\circ.8$ having Gaussian vertical distribution with a
half thickness of $\pm 0^\circ.2$. The CMZ is embedded in the Central HI Zone
(CHZ), which is defined as an HI disc between $l\sim -2^\circ$ and
$+2^\circ.5$, $b=-0^\circ.5$ and $+0^\circ.5$. Based on the analysis, we
discuss the origin of CMZ and interstellar physics such as the volume filling
factors of molecular and HI gases inferred from the difference between $ f_{\rm
mol}^\Sigma$ and $f_{\rm mol}^\rho$.
