A female diagnosed as having immune thrombocytopenic purpura (ITP) was found to be simultaneously suffering from monoclonal gammopathy of undetermined significance (IgGλ). Urine Bence-Jones protein was negative. During the course, plasma cells accounted for 21.6% of the bone marrow. Based on these clinical features in our case, the second disease was diagnosed as multiple myeloma (MM). Both ITP and MM were successfully treated with corticosteroids, bortezomib, lenalidomide with dexamethasone and eltrombopag olamine. MM with ITP may show the following features: 1) the great majority are IgG types, 2) λ chain types show marked light chain predominance when these two diseases appear simultaneously, 3) κ chain types are predominant in the cases with MM followed by ITP, and 4) MM cases with ITP are more often seen in Japan.