A heat exchanging system of a ventilating device includes: a heat exchanging housing communicating with an outdoor air passage and an indoor air passage; a rotational shaft rotatably supported at one side of the case of a ventilating device; and a heat exchanging elements mounted at an outer circumferential surface of the rotational shaft at regular intervals, and performing a heat-exchanging operation of outdoor air and indoor air while being rotated by a wind force of outdoor air passing the outdoor air passage and a blow force of indoor air passing the indoor air passage. Thus, any additional driving unit is not necessary to rotate the heat exchanging system, so that a fabrication cost can be reduced, the overall size of the ventilating device can be reduced, and a noise and vibration can be reduced.