The present invention is a diagnosing method for oil-filled electrical equipment for diagnosing a degree of risk with regard to occurrence of abnormality due to copper sulfide generation in oil-filled electrical equipment, and the method includes a first step of detecting specific compounds contained in insulating oil in said oil-filled electrical equipment, a second step of evaluating a possibility of copper sulfide generation at a dangerous part leading to dielectric breakdown in said oil-filled electrical equipment in accordance with a detection result obtained in said first step, and a third step of diagnosing a degree of risk with regard to occurrence of abnormality in said oil-filled electrical equipment in accordance with an evaluation result obtained in said second step. Said specific compounds include dibenzyldisulfide and 2,6-di-t-butyl-p-cresol.