Patients with the coronavirus disease 2019 (COVID-19) sometimes develop refractory respiratory failure and may require venovenous extracorporeal membrane oxygenation (VV-ECMO). It is known that the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is sometimes present in the blood of COVID-19 patients. VV-ECMO is often used for several weeks, and plasma leaks can occur, albeit rarely. Hence, in terms of infection control, a concern is that SARS-CoV-2 may leak from the gas outlet port of the oxygenator during ECMO support of critically ill COVID-19 patients. The aim of this study was to clarify whether SARS-CoV-2 leaks from the oxygenator during ECMO support. Five patients with critical COVID-19 pneumonia were placed on VV-ECMO. Silicone-coated polypropylene membrane oxygenators were used in the ECMO circuits for these patients. SARS-CoV-2 ribonucleic acid (RNA) was measured by quantitative reverse transcription polymerase chain reaction in serum and at the gas outlet port of the ECMO circuit at the time of circuit replacement or liberation from ECMO. SARS-CoV-2 RNA was detected in the gas outlet port of the ECMO circuit for three of the five patients. None of the medical staff involved in the care of these five patients has been infected with COVID-19. In conclusion, SARS-CoV-2 could leak to the gas outlet port of the ECMO circuit through silicone-coated polypropylene membranes during ECMO support of critically ill COVID-19 patients.