A window glass dew-point compartment interior absolute humidity Xrmax is acquired, and a temporary compartment interior set absolute humidity Xset (temporary Xset) is determined from a compartment interior set temperature Tset. A smaller one of Xrmax and (temporary Xset) is used as a compartment interior set absolute humidity Xset. An outlet air required absolute humidity Xao required to obtain Xset is calculated taking into account the humidity of exterior air introduced into the passenger compartment. Using Xao and an outlet air required temperature Tao required to obtain Tset, a control temperature Tea for controlling a compressor is determined.