  We present an unbiased large-scale (9 deg$^2$) CN ($N$=1-0) and C$^{18}$O
($J$=1-0) survey of Cygnus-X conducted with the Nobeyama 45m Cygnus-X CO
survey. CN and C$^{18}$O are detected in various objects towards the Cygnus-X
North and South (e.g., DR17, DR18, DR21, DR22, DR23, and W75N). We find that
CN/C$^{18}$O integrated intensity ratios are systematically different from
region to region, and are especially enhanced in DR17 and DR18 which are
irradiated by the nearby OB stars. This result suggests that CN/C$^{18}$O
ratios are enhanced via photodissociation reactions. We investigate the
relation between the CN/C$^{18}$O ratio and strength of the UV radiation field.
As a result, we find that CN/C$^{18}$O ratios correlate with the far-UV
intensities, $G_0$. We also find that CN/C$^{18}$O ratios decrease inside
molecular clouds, where the interstellar UV radiation is reduced due to the
interstellar dust extinction. We conclude that the CN/C$^{18}$O ratio is
controlled by the UV radiation, and is a good probe of photon-dominated
regions.
