A 69-year-old man was admitted with neck muscle weakness, symmetric proximal muscle weakness, skin rash and elevated serum creatine kinase levels. Muscle biopsy showed perifascicular necrosis and perimysial alkaline phosphatase activity. Chest CT revealed interstitial lung disease and colorectal cancer was diagnosed on colonoscopy. He was serologically positive for anti-EJ antibody, leading to the diagnosis of antisynthetase syndrome (ASS). After laparoscopic low anterior resection of the rectum, he received intravenous methylprednisolone (1,000 mg/d for 3 days) followed by oral prednisolone (50 mg/d). Although his muscle weakness improved after corticosteroid therapy, he developed pericardial effusion with resultant asymptomatic hypotension and arrhythmia possibly due to pericarditis. Corticosteroid monotherapy was insufficient to control the disease, and, we decided to use oral cyclosporin concurrently. After this combined therapy started, pericardial effusion and arrhythmia were improved. We should keep in mind that pericarditis can occur in patients with anti-EJ antibody-positive ASS, and early combined therapy with corticosteroid and immunosuppressive drugs for ASS may improve the patient's prognosis.