  Molecular gas in external galaxies is a subject of crucial importance for
observational and theoretical studies of galaxy formation. Compact molecular
gas around an active galactic nuclei (AGN) is expected to be an energy budget
of AGN and/or nuclear starburst. Recent observational studies suggest that line
ratios in millimeter and submillimeter band may be a good tool to reveal the
long-standing question on the origin of activity -- AGN or nuclear starburst.
We have constructed a powerful "telescope" of theory, three-dimensional nonLTE
line transfer code, preceding the high resolution and sensitivity observations
such as ALMA.
