A method is for diagnosing a rolling device comprising an outer member, an inner member, and a rolling element. The method includes 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 of the electric circuit when the alternating voltage is applied to the electric circuit, and calculating a lubrication film thickness and a metallic contact ratio between the outer member and the rolling element and/or between the inner member and the rolling element, based on a measured impedance and a measured phase.