A 66-year old man with type 2 diabetes mellitus has maintained good plasma glucose level with insulin therapy. His diabetic nephropathy was classified in G3aA2 stage of chronic kidney disease. He was administrated flurbiprofen after gastrectomy because of gastric cancer, and after that his urinary protein creatinine ratio was increased, in line with increase in both U-NAG and U-β2MG levels. We diagnosed NSAIDs-induced nephrotic syndrome to start predonisolon. After starting steroid therapy, proteinuria and renal tubular injury were rapidly improved. When proteinuria is suddenly increased in the course of diabetes mellitus, kidney diseases other than diabetic nephropathy should be considered.