We report a case of atrial fibrillation with frequent monomorphic atrial premature contractions (APCs) in which P-wave morphology was negative in II, III, and aVF and positive in I and aVL leads. After pulmonary vein isolation using a cryoballoon, high-resolution electroanatomical mapping revealed ectopic beats from the posteroseptal area within the inferior vena cava (IVC) with concealed bigeminy. Catheter ablation just above the ectopic focus eliminated the APC with residual dissociated activity inside the IVC. This is the first report to reveal continuous ectopic bigeminy definitely originating from the tiny area within the IVC by detailed three-dimensional mapping.