A 67-year-old man was referred to our hospital with anorexia. As a result of detailed examination, he was diagnosed with small cell lung cancer. Additionally, gait impairment from lower limb muscle weakness, elevation of anti-P/Q-type voltage-gated calcium channel antibodies, and a waxing phenomenon during a 30-Hz stimulation on repetitive stimulation test led to a diagnosis of Lambert-Eaton myasthenic syndrome. Four cycles of platinum doublet and atezolizumab therapy resulted in a complete response. His muscle symptoms remained unchanged. Maintenance atezolizumab still continues today without tumor relapse or worsening of muscle symptoms.