Bi-ion hybrid resonant frequency f_bi-ion is a characteristic frequency between two ion species. For the quasi-perpendicular wave propagation, the polarization change occurs near f_bi-ion, but the wave frequency is almost unchanged. f_bi-ion is not commonly observed in the spectrum since such polarization change occurs in a narrow frequency range. This study reports a bi-ion hybrid resonant frequency observation with a clear magnetic field polarization change from the compressional component dominance to the transverse component dominance. The determination of such a frequency is valuable for estimating ion concentrations, which is a critical parameter in space plasma. The ion concentration usually cannot be measured directly due to spacecraft charging, which often prevents cold ions from reaching detectors. The bi-ion hybrid resonant frequency is directly related to the ion concentration and, thus, can be used to infer the ion concentration. Using the bi-ion hybrid resonant frequency, we can determine the ranges of ion concentrations for each species in proton-helium-oxygen plasma and their one-to-one relation. Combining the observed proton, helium ion, and oxygen ion distributions, we further narrow down the range of ion concentrations.