  We present large scale observations of C18O(1-0) towards four massive star
forming regions for MON R2, S156, DR17/L906 and M17/M18. The transitions of
H2CO(110-111), C18O(1-0) and 6 cm continuum were compared towards the four
regions. Analysis of observation and Non--LTE model results shows that the
brightness temperature of the formaldehyde absorption line is strongest in
background continuum temperature range of about 3 - 8 K. The excitation of the
H2CO absorption line is affected by strong background continuum emission. From
the comparison of H2CO and C18O maps, we found that the extent of H2CO
absorption is broader than that of C18O emission in the four regions. Except
for the DR17 region, the H2CO absorption maximum is located at the same
position with the C18O peak. The good correlation between intensities and
widths of H2CO absorption and C18O emission lines indicate that the H2CO
absorption line can trace dense and warm regions of the molecular cloud.
Finding that N(H2CO) was well correlated with N(C18O) in the four regions and
that the average column density ratio is <H2CO/N(C18O)> ~ 0.03.
