PURPOSE: To prevent an occurrence of dew and freezing by a method wherein a heater for preventing an occurrence of dew and freezing and an indoor discharged air heater are arranged in the heat exchanger and ventilation device.

CONSTITUTION: An outddor air feeding passage 5 is provided with a heater 12 for preventing an occurrence of dew qnd freezing and a thermostat 13 for performing an automatic operation of the heater and further an outdoor air feeding pipe 14 is connected to the former passage. In door air discharging pipe 15 is connected to the primary stage indoor air discharging passage 7, the primary stage air feeding passage 8 is provided with the heater 17 for heating the indoor discharged air and the thermostat 18 for operating the heater at the set temperature and further the indoor air feeding pipe 19 is connected to the device. Under the operation of the air feeding blower 16 and the air discharging blower 10, the air is fed in the direction indicated by an arrow and when the temperature is decreased down to a value less than the predetermined temperature in the thermostat 13, the thermostat 8 is operated and the dew formation and freezing preventive heater 12 are operated.