A heat exchanger of a ventilating system includes a plurality of heat exchange plates which are laminated at a predetermined interval, a first heat exchange portion which is positioned among the heat exchange plates and through which outdoor air passes and a second heat exchange portion which is positioned among the heat exchange plates in turn with the first heat exchange portion and through which indoor air passes. The first and second heat exchange portions are composed of a plurality of heat exchange members having a predetermined length, and an inner surface of one heat exchange member and an outer surface of the other heat exchange member are aligned to face each other. Accordingly, the heat exchanger can improve heat transfer performance by restraining development of a boundary layer by improving air flowing structure of the heat exchanger.