A 39-year-old man who had been diagnosed with immunoglobulin A (IgA) nephropathy underwent renal transplant from his father. The operation was performed under cyclosporine, prednisolone and mizoribine treatment. Renal function was stable following transplantation, but proteinuria ranged between 1 g/day and 3 g/day. Protocol biopsy 1 year after transplantation revealed membranous glomerulonephritis, with IgG and C3 deposits under immunofluorescence, and subepithelial deposits detected on electron microscopy. The patient was treated by limiting protein intake, controlling blood pressure and administering candesartan. Proteinuria decreased from 5.6 g/day to 1 g/day, but a graft biopsy was performed 2 years after transplantation because of a slightly increasing creatinine level. There was no sign of rejection, and IgG and C3 deposits observed under immunofluorescence had decreased. After the graft biopsy, the creatinine level was stable and proteinuria decreased to 0.7 g/day. In conclusion, de novo nephropathy such as membranous glomerulonephritis should also be considered a possible cause of proteinuria following renal transplantation.