PURPOSE: To improve the increase of the blower noise and the excessive heating of the blower on the front seat side while the heating performance at the rear seat side is improved during the warm-up operation.

CONSTITUTION: In determining the air volume pattern of the total air volume of the air volume from an air outlet on the front seat side and that on the rear seat side during the warm-up operation, the total air volume is expressed by the air volume pattern A including the first control line (α-δ) indicated by the two-dot chain line where the air volume is gradually reduced from the maximum air volume WH according to the drop of the target diffused air temperature, and the second control line (δ-β) which is continuous to the first control line (α-δ) and rapidly reduced from the first control line (α-δ) and reaches the regular minimum air volume WL. The air volume pattern B of the air outlet on the front seat side and the air volume pattern C of the air outlet on the rear seat side are provided with the first and second control lines similar to the total air volume pattern A.