A 45-year-old man presented with moderate proteinuria and hematuria. A renal biopsy showed mesangial/endocapillary proliferative glomerulonephritis, linear deposition of monoclonal immunoglobulin γ1 CH2 heavy chain along glomerular and tubular basement membranes (GBMs and TBMs), granular deposition of κ light chain within the mesangial area, and continuous linear deposits of finely granular electron-dense materials along GBMs and TBMs. Dual immunostaining showed essentially discrete glomerular localization of γ1 CH2 heavy chain and κ light chain. Monoclonal protein was not detected in urine and serum. A bone marrow aspiration showed no abnormalities. Steroid therapy led to the improvement of proteinuria and hematuria. We would classify this case as a new variant of monoclonal immunoglobulin deposition disease, light chain/heavy chain deposition disease. In contrast with light and heavy chain deposition disease, the remarkable characteristics of this variant are separate deposition of monoclonal heavy chain and light chain, deposition of largely deleted γ heavy chain lacking the CH1 domain, and good response to steroid therapy.