  An imaging survey of CO(1-0), HCN(1-0), and HCO$^+$(1-0) lines in the centers
of nearby Seyfert galaxies has been conducted using the Nobeyama Millimeter
Array and the RAINBOW interferometer. Preliminary results reveal that 3
Seyferts out of 7 show abnormally high HCN/CO and HCN/HCO$^+$ ratios, which
cannot occur even in nuclear starburst galaxies. We suggest that the enhanced
HCN emission originated from X-ray irradiated dense obscuring tori, and that
these molecular line ratios can be a new diagnostic tool to search for ``pure''
AGNs. According to our HCN diagram, we suggest that NGC 1068, NGC 1097, and NGC
5194 host ``pure'' AGNs, whereas Seyfert nuclei of NGC 3079, NGC 6764, and NGC
7469 may be ``composite'' in nature.
