A 49-year-old woman presented with progressive muscle weakness of the limbs and dysphagia. Her past and family medical history were unremarkable and she did not take statins or any other medications. Laboratory tests showed that serum levels of creatine kinase were elevated (13,565 IU/l) and anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) antibodies were detected in the serum. Other autoantibodies to the nuclear (ANA), RNP, aminoacyl-tRNA synthetases (ARS), and signal recognition particle (SRP) were negative. Pathological analysis of the left biceps muscle revealed minimal lymphocytic infiltration into the muscle fibers together with many necrotic and regenerated fibers, which corresponded to necrotizing myopathy. Abdominal CT and upper gastrointestinal endoscopy showed an advanced gastric cancer with lymph node metastasis. The patient was subsequently diagnosed with anti-HMGCR antibody-positive paraneoplastic necrotizing myopathy associated with advanced gastric cancer. The patient underwent radical surgery to remove the cancer and was initially treated with oral prednisolone and intravenous methylprednisolone pulse therapy; however, her symptoms worsened and she became bedridden. After an additional treatment with intravenous immunoglobulin (IVIg), she showed noticeable improvements in muscle strength and dysphagia and became ambulatory. This case and recent case-series studies suggest that anti-HMGCR antibody-positive necrotizing myopathy may be included in paraneoplastic syndrome and that physicians should screen for malignant tumors in patients with anti-HMGCR antibody-positive necrotizing myopathy. Moreover, IVIg can be a useful therapy in patients with anti-HMGCR antibody-positive paraneoplastic necrotizing myopathy who show refractoriness to tumor resection and corticosteroid therapies.