High-quality cardiopulmonary resuscitation (CPR) is crucial for survival from cardiac arrest. However, various chest compression-associated injuries have been reported. Internal mammary artery (IMA) injury is one of the rare complications after CPR, and most of cases include rib and sternum fractures. In this report, we describe a rare case of IMA injury without chest wall fractures after CPR. An 85-year-old man with a history of acute myocardial infarction 2 weeks prior visited to our hospital for sustained ventricular tachycardia (VT). After admission, sustained VT requiring CPR occurred several times. Emergency coronary angiogram revealed 90% stenosis at the left anterior descending artery. Hence, emergency percutaneous coronary intervention (PCI) was performed. During the PCI, blood gas analysis showed decreasing serum hemoglobin levels. Contrast computed tomography revealed hemothorax and extravasation at the branch of the right IMA without chest wall fractures. The patient's deteriorating hemodynamic condition precluded thoracotomy or embolization to stop the bleeding. The patient died on the next day of hospitalization. IMA injury can occur after CPR, regardless of chest wall fractures and can be fatal without early diagnosis. For an emergency physician, IMA injury should be considered as a cause of unknown anemia after CPR.