A 67-year-old man was diagnosed with severe coronary artery stenosis in the left main trunk and two vessels in January 2009. Coronary artery bypass graft surgery was performed. After surgery, coronary angiograms revealed a severe stenosis of the left anterior descending artery at the distal portion of the anastomosis site. Percutaneous coronary intervention (PCI) to the culprit lesion was performed. During the operation, extravascular leakage of contrast agent was observed, and perforation of the coronary artery due to ballooning was suspected. The patient was referred to our hospital. A cystic mass (size 50×44×32 mm) was shown on the anterior interventricular sulcus by echocardiography. At follow-up echocardiography 20 days after the first detection, the mass had markedly increased in size to 57×47×70 mm. The risk of rupture was high, so an operation for closure of the coronary artery pseudoaneurysm was performed. Echocardiography was performed 1 year after surgery, at which time the mass had disappeared. We experienced a coronary artery pseudoaneurysm with penetration of the coronary artery due to PCI. Echocardiography was useful for evaluating the size of the pseudoaneurysm over time, allowing us to avoid the risk of rupture.