PROBLEM TO BE SOLVED: To provide a protective cover having an erroneous fitting preventing structure for preventing the cover from fitting to an artificial lung body in an erroneous posture.SOLUTION: A protective cover 23 is attached to cover at least a part of an artificial lung body 22 having a gas exchange chamber in the inside. The protective cover includes first and second frames 24, 25, one of which is arranged on the front surface of the artificial lung and the other on the rear surface thereof. The first frame has a pair of right-left fitting pieces 26, 27 each extending to the second frame when the cover is attached to the artificial lung body, which are formed on upper and lower ends, respectively. Engagement claws 28 are formed on the front ends of the engagement portions 26a, 27a formed by some of the fitting pieces. The second frame has shoulder portions 29 on upper and lower ends, respectively. A pair of right-left fitting holes for allowing the fitting pieces of the first frame to fit thereto are formed on the shoulder portions, and engagement holes 30, 31 enabling the engagement claws to be engaged therewith are formed at positions corresponding to the engagement claws in the fitting holes. At least either an interval on the outside edge of the fitting pieces or an interval on the inside edge thereof is different between the upper end portion and the lower end portion.
