PURPOSE: To perform optimum mode door control without using complicated control logic.

CONSTITUTION: Fuzzy inference is carried out for the opening degree θ of a blow-off mode door for adjusting the ratio between upper and lower blow-off volumes with the use of a total signal T corresponding to a thermal load in the passenger compartment, a solar radiation value Qs, an ambient air temperature Ta and a set temperature Tset as pre-condition variables. In the fuzzy inference, at a first fuzzy inference, mode door opening degrees θ m are obtained for respective fuzzy rules in accordance with the precondition variables T, Qs, and the logical OR thereof is obtained (Step 3). Then, at a second fuzzy inference, mode door opening degrees θ m are obtained for respective rules in accordance with the precondition variables Ta, Tset, and the logical OR thereof is obtained. Further, the logical OR of the results of the two fuzzy inferences and the gravitational center thereof is obtained. Then, using the gravitational center as an optimum result of the fuzzy inference (Step 105), the opening degree of the mode door is controlled so that the mode door opening as the result of the inference is obtained (Step 105).