  We performed correlation analyses between the $^{12}$CO and $^{13}$CO $J=$1-0
line intensities in order to derive the variability of the CO-to-H$_2$
conversion factor ($X_{\rm CO, iso}$) in the central molecular zone (CMZ) of
our Galaxy. New high-resolution $X_{\rm CO, iso}$ maps at a resolution of $\sim
30$" and the longitude-velocity diagram (LVD) at resolution $\sim 30$" $
\times\ 2$ km s$^{-1}$ are presented using the $^{12}$CO and $^{13}$CO archival
survey data obtained by the Nobeyama 45 m telescope. We revealed the variation
of $X_{\rm CO, iso}$ in the CMZ within the range of $X_{\rm CO, iso} \sim
(0.2-1.3) \times 10^{20}\ {\rm cm^{-2}\ (K\ km\ s^{-1})^{-1}}$, if we assume
the normalization value of $0.59 \times 10^{20}\ {\rm cm^{-2}\ (K\ km\
s^{-1})^{-1}}$. The mean value is obtained as $X_{\rm CO, iso} = (0.48 \pm
0.15) \times 10^{20}\ {\rm cm^{-2}\ (K\ km\ s^{-1})^{-1}}$ in the CMZ of our
Galaxy.
