The incidence of second primary malignancies (SPMs) in Japanese patients with myeloma or myeloma-related diseases was studied by using the Kansai Myeloma Forum (KMF) database registered from November 2012 to March 2015. We studied 1,571 cases. Hematologic malignancies were documented in 10 patients, and solid tumors in 36 during this period. The cumulative 5-year incidence was estimated to be 1.0% for hematological malignancies and 3.7% for solid tumors. In the patients with smoldering myeloma or MGUS without treatment, solid tumors but not hematologic malignancies developed, though the cumulative incidence of each malignancy did not differ significantly from that in patients receiving treatment. Although statistical analysis showed that treatment with melphalan, bortezomib, lenalidomide, or thalidomide had no effect on the occurrence of hematological malignancies, lenalidomide administration was more frequent in the patients with solid tumors. To evaluate the SPMs in myeloma or myeloma-related diseases more accurately, accumulation of a larger number of patients and longer observation are needed.