  In observational studies of infrared dark clouds, the number of detections of
CO freeze-out onto dust grains (CO depletion) at pc-scale is extremely limited,
and the conditions for its occurrence are, therefore, still unknown. We report
a new object where pc-scale CO depletion is expected. As a part of Kagoshima
Galactic Object survey with Nobeyama 45-m telescope by Mapping in Ammonia lines
(KAGONMA), we have made mapping observations of NH3 inversion transition lines
towards the star-forming region associated with the CMa OB1 including IRAS
07077-1026, IRAS 07081-1028, and PGCC G224.28-0.82. By comparing the spatial
distributions of the NH3 (1,1) and C18O (J=1-0), an intensity anti-correlation
was found in IRAS 07077-1026 and IRAS 07081-1028 on the ~1 pc scale.
Furthermore, we obtained a lower abundance of C18O at least in IRAS 07077-1026
than in the other parts of the star-forming region. After examining high
density gas dissipation, photodissociation, and CO depletion, we concluded that
the intensity anti-correlation in IRAS 07077-1026 is due to CO depletion. On
the other hand, in the vicinity of the centre of PGCC G224.28-0.82, the
emission line intensities of both the NH3 (1,1) and C18O (J=1-0) were strongly
detected, although the gas temperature and density were similar to IRAS
07077-1026. This indicates that there are situations where C18O (J=1-0) cannot
trace dense gas on the pc scale and implies that the conditional differences
that C18O (J=1-0) can and cannot trace dense gas are unclear.
