In view of the increase in the number of cases with nephropathy in aged animals of the cat family, the invention provides a marker for early diagnosis of feline nephropathy which can substitute for the conventional complicated methods of diagnosing nephropathy such as blood examination, kidney radiography using a contrast medium, and ultrasonic image analysis. It was found that cats with nephropathy show markedly lowered levels of excretion of a novel protein (named cauxin) with a molecular weight of 70 K, which is excreted into urine in high concentrations in normal adult cats showing no clinical abnormality. When the protein is used as a marker in diagnosing feline nephropathy, feline nephropathy can be detected in a simple and exact manner.