A method for diagnosing a rolling device comprising an outer member, an inner member, and a rolling element, the method comprising: applying an alternating voltage to an electric circuit configured by the outer member, the rolling element and the inner member; measuring an impedance and a phase (θ) of the electric circuit when the alternating voltage is applied to the electric circuit; and characterized by: calculating a lubrication film thickness (h) and a metallic contact ratio (α) between the outer member and the rolling element and between the inner member and the rolling element, based on the measured impedance and the measured phase, wherein the lubrication film thickness (h) and the metallic contact ratio (α) are derived by equations and: h = ωε 1 S − n 2 Z sin θ − ωL 2 + Z cos θ − R 2 Z sin θ − ωL + R 20 Z cos θ − R Z sin θ − ωL − Z cos θ 1 − R Z sin θ 1 − ωL α = 2 n R 20 Z sin θ − ωL 2 + Z cos θ − R 2 Z cos θ − R − Z cos θ 1 − R Z sin θ − ωL Z sin θ 1 − ωL wherein: ω: frequency of alternating voltage ε 1: dielectric constant of lubricant S: average value of areas of respective contact ellipses when each contact region is approximated to a contact ellipse n: the number of the rolling elements of the bearing device Z: impedance of the entire electric circuit θ: phase R 20: resistance of a metallic contact part in state where the lubrication film is not completely formed θ 1: phase in state where the lubrication film is completely formed L: inductance L connected in series with the rolling device R: resistance R connected in series with the rolling device. applying an alternating voltage to an electric circuit configured by the outer member, the rolling element and the inner member; measuring an impedance and a phase (θ) of the electric circuit when the alternating voltage is applied to the electric circuit; and characterized by: calculating a lubrication film thickness (h) and a metallic contact ratio (α) between the outer member and the rolling element and between the inner member and the rolling element, based on the measured impedance and the measured phase, wherein the lubrication film thickness (h) and the metallic contact ratio (α) are derived by equations and: h = ωε 1 S − n 2 Z sin θ − ωL 2 + Z cos θ − R 2 Z sin θ − ωL + R 20 Z cos θ − R Z sin θ − ωL − Z cos θ 1 − R Z sin θ 1 − ωL α = 2 n R 20 Z sin θ − ωL 2 + Z cos θ − R 2 Z cos θ − R − Z cos θ 1 − R Z sin θ − ωL Z sin θ 1 − ωL wherein: ω: frequency of alternating voltage ε 1: dielectric constant of lubricant S: average value of areas of respective contact ellipses when each contact region is approximated to a contact ellipse n: the number of the rolling elements of the bearing device Z: impedance of the entire electric circuit θ: phase R 20: resistance of a metallic contact part in state where the lubrication film is not completely formed θ 1: phase in state where the lubrication film is completely formed L: inductance L connected in series with the rolling device R: resistance R connected in series with the rolling device. calculating a lubrication film thickness (h) and a metallic contact ratio (α) between the outer member and the rolling element and between the inner member and the rolling element, based on the measured impedance and the measured phase, wherein the lubrication film thickness (h) and the metallic contact ratio (α) are derived by equations and: h = ωε 1 S − n 2 Z sin θ − ωL 2 + Z cos θ − R 2 Z sin θ − ωL + R 20 Z cos θ − R Z sin θ − ωL − Z cos θ 1 − R Z sin θ 1 − ωL α = 2 n R 20 Z sin θ − ωL 2 + Z cos θ − R 2 Z cos θ − R − Z cos θ 1 − R Z sin θ − ωL Z sin θ 1 − ωL wherein: ω: frequency of alternating voltage ε 1: dielectric constant of lubricant S: average value of areas of respective contact ellipses when each contact region is approximated to a contact ellipse n: the number of the rolling elements of the bearing device Z: impedance of the entire electric circuit θ: phase R 20: resistance of a metallic contact part in state where the lubrication film is not completely formed θ 1: phase in state where the lubrication film is completely formed L: inductance L connected in series with the rolling device R: resistance R connected in series with the rolling device. ω: frequency of alternating voltage ε 1: dielectric constant of lubricant S: average value of areas of respective contact ellipses when each contact region is approximated to a contact ellipse n: the number of the rolling elements of the bearing device Z: impedance of the entire electric circuit θ: phase R 20: resistance of a metallic contact part in state where the lubrication film is not completely formed θ 1: phase in state where the lubrication film is completely formed L: inductance L connected in series with the rolling device R: resistance R connected in series with the rolling device.