Introduction: We experienced a case of lung cancer in which arthrocentesis and radiation were effective in treating pain caused by malignant joint fluid. Case: An 80-year-old man was referred to our hospital because of right shoulder pain and difficulty in raising his right hand. Whole body computed tomography (CT) showed right lung tumor, multiple liver metastases and multiple bone metastases. There were also bone metastases in the right scapula and joint fluid accumulation in the right shoulder joint. He was irradiated for bone metastases in the right scapula, but the pain remained, although it had not worsened. Therefore, an arthrocentesis of the right shoulder joint was performed and pain was alleviated. In addition, the diagnosis of non-small cell carcinoma was made by joint fluid cytology. A biopsy was also taken from the right lung tumor, and as the histological diagnosis was similar to that of the joint fluid, chemotherapy was started. CT after the start of treatment showed a decrease in the right shoulder joint fluid and no progression of right scapular metastases. Conclusion: In case of malignant joint fluid associated with bone metastases, a severe prognosis is expected, but arthrocentesis and irradiation can reduce pain.