We present the results of systematic millimeter interferometricobservations of luminous infrared galaxies (LIRGs), using the NobeyamaMillimeter Array.We observed HCN (J = 1–0) and HCO+ (J = 1–0) emission linessimultaneously, and derived their brightness-temperature ratios, toinvestigate whether the observed ratios are similar to those found inAGN-dominated nuclei or starburst galaxies. LIRGs with (without) luminous buried AGN signatures in our infraredspectra 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 theenhanced HCN abundance, and/or infrared radiative pumping of the HCNmolecule, in the close vicinity of an AGN.