Cardiac erosion is a rare serious complication following Amplatzer septal occluder (ASO) placement for atrial septal defect. Although multiple risk factors have been found, a useful predictor to prevent it has not been established yet. In 12 patients who developed erosion between 2005 and 2016 in Japan, we retrospectively observed patients’ characteristics and transesophageal echocardiography findings immediately before and after ASO placement and at erosion onset. We compared risk factors of erosion, including absent aortic rim, device size/body weight ratio, device shape, or Valsalva sinus wall deformation pressed by either disk-edge, the maximum depth of which from the standard curve of the Valsalva wall was defined as Dent, between the 12 patients and 95 patients with Valsalva sinus wall deformation immediately after placement who did not develop erosion for 5 ± 3 years. Of the 12 patients, nine developed pericardial effusion with eight cardiac tamponade and three aorta-atrium fistula; all were surgically rescued. Surgical findings revealed that erosion in all patients occurred at the right and/or left atrial roof beside the Valsalva in the non-coronary cusp on which the disk-edge seemed to be pressing. The mean Dent immediately after the placement in patients with erosion was significantly deeper than without (2.48 ± 0.32 vs. 1.28 ± 0.38; p < 0.001). There were no differences in the other risk factors between the two groups. Dent is believed to be a useful indicator of erosion development after ASO placement. If Dent is > 2.0 mm, it is desirable to change the size or to replace the device.