A 73-year-old male with malignant pleural mesothelioma developed muscle weakness of the lower limbs, ptosis, and dysarthria with elevation of serum creatine phosphokinase (CPK) levels after the first course of combination therapy with nivolumab and ipilimumab. He was diagnosed with myasthenia gravis with myositis and was subsequently started on steroid therapy. After that, he developed rapidly worsening type 2 (hypercapnic) respiratory failure caused by exacerbation of myasthenia gravis and required intubation and mechanical ventilation. Although plasma exchange therapy was performed, respiratory failure persisted, and intravenous high-dose immunoglobulin therapy was added. We performed plasma exchange therapy and intravenous high-dose immunoglobulin therapy with steroid therapy. After that, his respiratory status improved progressively and he could eventually be weaned from the ventilator and discharged home. We consider this to be a valuable case in which the life of a patient with malignant pleural mesothelioma was saved from myasthenia gravis with myositis, which developed and rapidly deteriorated after administration of nivolumab and ipilimumab.