A 71-year-old man receiving chemotherapy for non-small cell lung cancer noticed diplopia, left ptosis, and bilateral lower leg pain three weeks after initial treatment with the immune checkpoint inhibitor pembrolizumab. Creatine kinase levels had increased to 6,001 U/L and magnetic resonance imaging of the thigh revealed inflammation of the right vastus medialis. He tested negative for autoantibodies related to inflammatory myopathy. We diagnosed him with pembrolizumab-related myasthenia gravis-like myositis based on clinical symptoms, elevation of muscle enzymes, and the areas of muscle inflammation revealed on magnetic resonance imaging. Treatment with prednisolone rapidly ameliorated serum creatine kinase levels. We administered intravenous immunoglobulin, steroid therapy, and plasma exchange therapy. His symptoms gradually improved and he was discharged on the 85th day after admission.