In monoclonal IgG cryoglobulinemia, two types of crystallization have been demonstrated with electron microscopy at high magnification. In contrast, little information is available on well-defined crystallization in cases of monoclonal IgM cryoglobulinemia. We present a case of pure monoclonal IgM kappa cryoglobulinemia and rapidly progressive glomerulonephritis occurring in a 75-year-old woman. We detected unequivocal formation of fibrillar crystal structure within intraglomerular macrophages on electron microscopy: the structure accords with that in a case of monoclonal IgG cyroglobulinemia described in the literature.