The present invention is related to a method of diagnosing an oil-immersed electrical apparatus by an assessment of a state of deterioration of the oil-immersed electrical apparatus, in which the oil-immersed electrical apparatus includes an insulating oil and an insulating paper and the insulating paper contains an amine compound. This method includes performing a quantitative analysis of a marker substance contained in the insulating oil and assessing a state of deterioration of the oil-immersed electrical apparatus based on a quantified value of the marker substance obtained by the quantitative analysis. The marker substance is a degradation product of cellulose, and this degradation product of cellulose does not chemically react with a deterioration product of the amine compound.