A 62-year-old Japanese man presented a hard and painful intramuscular mass in the right upper arm during the chemotherapy for lung squamous cell carcinoma. Initially, this mass containing fluid accumulation was treated by radiotherapy and antibiotics as a muscle metastasis suspected to be complicated with local infection. However, because the swelling and pain of his right arm did not improve, he underwent a surgical debridement of the mass. These local treatments succeeded in relieving the patient's symptoms for a while. However, after temporary remission, the recurrence tumor developed the paralysis of right radial nerve and ulnar nerve in his upper arm. Despite further combined therapy including drainage, additional radiotherapy, and chemotherapy, paralysis made his performance status deteriorated. He was eventually discontinued aggressive treatment due to worsened general condition. We herein report a case of lung cancer followed unusual course due to muscle metastasis in the triceps muscle. Because the paralysis caused by muscle metastasis can be the factor to deteriorate the performance status of patient, the combined therapy including antibiotics, debridement, radiotherapy and chemotherapy as early as possible should be considered to avoid its risk.