A 55-year-old man presented to our department with diarrhea, weight loss, fatigability, and polyarthralgia. Blood tests revealed elevated soluble interleukin-2 receptor levels and IgG-type M protein positivity, without any findings that were suggestive of collagen disease. After computed tomography (CT) detected enlarged lymph nodes in the abdominal para-aortic region, lymphoma was suspected. CT-guided needle biopsy of the lymph node did not help to achieve a definitive diagnosis; however, a bone marrow test showed the pathological features of B-cell lymphoma. A genetic examination detected a MYD88 L265P mutation; the mutation analysis was valuable in diagnosing lymphoplasmacytic lymphoma in a IgM-type M protein-negative patient.