We present the results of systematic millimeter interferometric observations of luminous infrared galaxies (LIRGs), using the Nobeyama Millimeter Array. We observed HCN (J = 1–0) and HCO+ (J = 1–0) emission lines simultaneously, and derived their brightness-temperature ratios, to investigate whether the observed ratios are similar to those found in AGN-dominated nuclei or starburst galaxies. LIRGs with (without) luminous buried AGN signatures in our infrared spectra tend to show high (low) HCN (J = 1–0) to HCO+ (J = 1–0) brightness-temperature ratios as seen in galaxy nuclei dominated by AGNs (starbursts). The high ratios in buried AGN candidates could be explained by the enhanced HCN abundance, and/or infrared radiative pumping of the HCN molecule, in the close vicinity of an AGN.