To provide a method for operating with minimum energy, while keeping a temperature and humidity, in an air conditioning process suitable for a constant temperature and humidity room such as an industrial clean room.

A method for air conditioning control of a clean room includes the steps of forecasting the change of outside air temperature and humidity from the weather data and an actual result value, and calculating a forecast ratio enthalpy and outside air forecast dew point from forecasted outside air forecast temperature and humidity. By this arrangement, the number of refrigerating machines is controlled which is necessary for the operation of an outdoor air-conditioning unit having a cooling coil. At the same time, when the outside air forecast dew-point or introduced outside air dew-point is lower than the dew-point supplied to a target space, the preset temperature of the cooling coil is changed to a temperature supplied to the target space. Further, when the outside air forecast temperature is lower than the supply temperature, the refrigerating machine is stopped.