A 40-year-old man was admitted to our hospital because of dysarthria, difficulty swallowing, double vision and weakness of both upper extremities. There were no detectable anti-AChR antibodies. He was diagnosed with seronegative myasthenia gravis (SNMG) based on a positive edrophonium test and positive waning on repetitive stimulation. Thereafter serological examination detected anti-muscle-specific kinase (MuSK) antibodies and he was diagnosed with anti-MuSK antibody-positive MG. Three years after the onset, the patient developed rapidly progressing respiratory failure and fever. He was diagnosed with aspiration pneumonia caused by swallowing difficulty. He was treated with mechanical ventilation, plasma exchange and antibiotics. Laboratory tests on admission also demonstrated nephrotic syndrome. Renal biopsy specimens showed diffuse thickening of the basement membrane by PAS and PAM stain, and granular immunofluorescent deposits of IgG4 along the glomerular capillary walls. Therefore, he was also diagnosed with membranous nephropathy in addition to anti-MuSK antibody-positive MG. MG is sometimes complicated with nephrotic syndrome, however there has been no report of anti-MuSK-antibody positive MG complicated with nephrotic syndrome. It has been reported that anti-MuSK-antibodies are IgG4 and that membranous nephropathy is suggested to be an IgG4 mediated disease. Our findings suggest that IgG4 may play an important role in the pathogenesis of our patient.