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 gas exchange performance, a heat exchange performance coefficient, a blood filling amount, and a blood side pressure loss are measured, they result in a performance factor between 1.5 and 2.5, wherein the performance factor is defined as (gas exchange performance×heat exchange performance coefficient)/(blood filling amount×blood side pressure loss).
