A hollow fiber membrane layer and the housing is accommodated in the housing, the hollow fiber membranes 31 having a gas exchange function is multiplicity of integration, as the gas flow path, an artificial lung, the gas flow to the lumen of the hollow fiber membranes 31 as a blood flow path upstream side of the road and the gas inlet portion respectively provided on the downstream side and a gas outlet, the outside of each hollow fiber membrane 31, the upstream side of the blood flow path and blood inflow respectively provided on the downstream side I and a blood outlet portion and the part. Each hollow fiber membrane 31 of the hollow fiber membrane layers are fixed to each other at one end portion and at the other end, ε the mean distance 31 between the hollow fiber membranes adjacent the portion that is fixed in the hollow fiber membrane [ When μm], was the OD [μm] the outer diameter of the hollow fiber membrane 31, which is 4.5 × ε 30 ≦ ε ≦ 60, OD ≦.人工肺は、ハウジングと、ハウジング内に収納され、ガス交換機能を有する中空糸膜31が多数本集積された中空糸膜層と、各中空糸膜31の内腔をガス流路として、ガス流路の上流側および下流側にそれぞれ設けられたガス流入部およびガス流出部と、各中空糸膜31の外側を血液流路として、血液流路の上流側および下流側にそれぞれ設けられた血液流入部および血液流出部とを備える。中空糸膜層の各中空糸膜31は、一端部および他端部においてそれぞれ互いに固定されており、中空糸膜層の固定された部位における隣り合う中空糸膜31同士の平均離間距離をε[μm]、中空糸膜31の外径をOD[μm]としたとき、30≦ε≦60、OD≦4.5×εである。
