An artificial lung has a gas exchanger portion with an external circulation-type gas exchanging hollow fiber membrane bundle and a heat exchanger portion with a heat exchanging element. Various design parameters of the gas exchanger and heat exchanger portions are configured to obtain a predetermined performance factor. In one embodiment, the design parameters are adjusted so that when a heat exchange performance coefficient of the artificial lung, a blood filling amount of the artificial lung, and a blood side pressure loss of the artificial lung are measured, they result in a performance factor equal to or greater than 1.5, wherein the performance factor is defined as heat exchange performance coefficient/(blood filling amount×blood side pressure loss).
