Recently, the early detection and the advances in therapy for malignant diseases have contributed to prolonged survival of patients, resulting in an increment of multiple primary malignancies. We describe a 55-year-old man, at the first presentation, with six malignancies over 14 years(malignant lymphoma, gastric cancer, ureteral cancer, small cell lung cancer, bladder cancer, and squamous cell lung cancer). A case of six primary malignancies is extremely rare and, as far as we know, this is the 16th case of its kind reported in Japan. The overlapping of many malignant diseases resulted in some difficulties with treatment. Whereas the ureteral cancer and small cell lung cancer were synchronous, considering the therapeutic duration of lung cancer, we proceeded with the operation for ureteral cancer and had to delay the start of chemotherapy for small cell lung cancer for more than one month. Moreover, dose intensity of the chemotherapy for the small cell lung cancer was limited by expectancy of augmented myelosuppression, due to the effect of prior chemotherapy for malignant lymphoma. However, a strong neutropenia-induced postoperative abdominal infection necessitated discontinuation of chemotherapy and treatment with radiotherapy alone. In addition, the therapies for the newly developed squamous cell lung cancer, the sixth malignancy, were also limited because of reduced lung function and myelopoiesis. In treatment or follow-up of patients with multiple primary malignancies, as opposed to those with a single malignant disease, the characteristics of other malignancies and the morbidities by preceding therapies must be considered.