The patient was a 74-year-old woman. An immune checkpoint inhibitor (pembrolizumab) was administered for recurrence after surgery for lung adenocarcinoma. Although the treatment was effective (partial response), arthralgia and myalgia appeared around 4 months after the treatment started, and her upper limb elevation and walking became impaired. Imaging examination revealed no recurrence of lung cancer or deformation in any joint. However, ¹⁸F-fluorodeoxyglucose-positron emission tomography-computed tomography (PET-CT) revealed multiple abnormal accumulations in the bilateral shoulders, hip joints, and lumbar spinous processes. A diagnosis of polymyalgia rheumatica (PMR) was made based on the negative test result for rheumatoid factor and increased blood sedimentation rate. After treatment with corticosteroids was initiated, the pain improved, and PET-CT reexamination 1 year later showed markedly diminishment of the abnormal accumulation in the joints. PMR as an immune-related adverse event is rarely reported. PET-CT was useful for the diagnosis and evaluation of PMR.