A 50-year-old man with paroxysmal atrial fibrillation underwent pulmonary vein isolation (PVI) using a cryoballoon. During the PVI procedure, a circular diagnostic catheter (CDC) was wedged deeply into the right PV (RPV). The lung tissues seemed to be tightening around the CDC. After various attempts to resolve the situation, the shaft of the CDC was completely torn, and the circular part remained in the RPV. At a follow-up of three months, the patient showed neither atrial fibrillation events nor clinical symptoms. We herein report a broken CDC shaft during PVI with a cryoballoon, which has not yet been reported.