Engine cooling water is charged to a flexible diaphragm member (3d) to expand the flexible diaphragm member (3d) so that the substantial sectional area of an air passage (3b) is decreased when a ventilation resistance, in an air conditioning casing (2) in a maximum heating or a foot mode, is increased. On the other hand, the engine cooling water is discharged from the flexible diaphragm member (3d) to deflate the flexible diaphragm member (3d) so that the substantial sectional area of the air passage (3b) is increased when a ventilation resistance, in the air conditioning casing (2) in maximum cooling or face mode, is decreased. Thus, the property of a fan (3) can be changed into a property suitable for foot mode having a large ventilation resistance, while heating air is being supplied.