A 83-year-old female patient was admitted to our hospital due to hematological manifestation of juvenile granulocytes and macrocytic anemia. Bone marrow (BM) examination revealed erythroid dysplasia and cytoplasmic blasts, and hence the patient was diagnosed with myelodysplastic syndrome with ring sideroblasts and with single lineage dysplasia (MDS-RS-SLD). Erythrocyte transfusion was performed as a supportive therapy, and there was a gradual increase in the number of blood cells. Therefore, BM re-examination was performed and it was confirmed that the number of megakaryocytes increased, so the patient's condition was determined as myelodysplastic/myeloproliferative neoplasms with ring sideroblasts and thrombocytosis (MDS/MPN with RS-T). Incidentally, gene mutation analysis showed CALR gene mutation. Thereafter, administration of hydroxycarbamide and anagrelide did not show adverse events and complications, and a good blood count control was obtained. Furthermore, it was also confirmed that an SF3B1 gene mutation is highly positive in MDS-RS. There was no report on CALR-mutant MDS/MPN in Japan, and it is a rare disease overseas.