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 nucleus (AGN) is expected to be an important part of the energy budget of the AGN and/or the nuclear starburst. Recent observational studies suggest that line ratios in the millimeter and submillimeter bands 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, a three-dimensional non-LTE line transfer code, preceding the high resolution and sensitivity observations such as ALMA.