The present invention relates to a method of diagnosing deterioration by comparing an index value which is obtained by dividing the relative harmonic content of each order of the current harmonics flowing into an electric motor and inverter by the total harmonic distortion of the current harmonics up to the predetermined order, with a criteria value which is obtained by multiplying a harmonic function of each order formed of the index value by a calculated value for diagnosis of each order found through calculation from the relative harmonic content of each order. In the method, the degrees of deterioration of the electric motor and inverter are distinguished from each other by weighting the criteria value, and the deteriorated part is determined by a specific harmonic order of the current harmonics.