PURPOSE: To provide an air conditioning device for vehicle which can purify air while ensuring a necessary air flow quantity, and the method thereof.

CONSTITUTION: In an air conditioning device for vehicle in which a filter 4 purifying passing air is provided in an air passage 11 between an intake door 3 and a fan 5, a ventilation changeover means 21 which takes the air in a car room or the air outside the car room in the air passage 11 without passing through the filter 4 is provided in the air passage 11 between the filter 4 and the fan 5. When the fan 5 is operated at the maximum air flow quantity, in addition to taking air in through the filter 4, a fixed quantity of air without passing through the filter 4 is partially taken in, and hence influence of the ventilation resistance of the filter 4 is lightened so as to ensure a necessary air flow quantity.