An external-air conditioning apparatus and a ventilation system are provided, the external-air conditioning apparatus includes a fin tube type heat exchanger and a housing that stores this heat exchanger. The housing includes a base body that serves as a back side of the apparatus body and a cover body that serves as a front side of the apparatus body, and the base body and the cover body are dividable and detachable in structure. The base body has a base portion, a partitioning portion, a connecting pipe portion, a heat-exchanger-holding structure portion, and a water receiving portion. The cover body is a structure in which a front wall portion and a three-side peripheral wall portion are formed integrally with each other, and a part in which the peripheral wall portion is not formed is an inlet for external air.