To miniaturize an air conditioner without impairing efficiency of temperature control of cooling water.

A cooling heat exchanger 34 and a heating heat exchanger 35 are arranged in series in an air blowing duct 37 from an air blower 32. Air is cooled by circulating the cooling water in the cooling heat exchanger 34 as a refrigerant, and the air is heated by circulating steam in the heating heat exchanger as a heating medium. These respective heat exchangers 34 and 35 are selectively used. The refrigerant or the heating medium of the unused heat exchanger is extracted from a coil of the respective heat exchangers 34 and 35. The air from the air blowing duct 37 is sent to an applying drier of a PS board manufacturing line, to dry a coating film at the proper temperature. When switching to cooling from heating and to the heating from the cooling, switching is quickly performed since there is no refrigerant or heating medium in the heat exchanger unused so far. The switching is similarly quickly performed since the heat exchanger used so far is switched to a new heat exchanger after extracting the refrigerant or the heating medium.