An air conditioning method of regulating temperature and humidity of process air to deliver it as supply air, the air conditioning method comprising: in an air conditioning system including: measuring equipment (33, 34, 35, 36, 49b, 28, 6, 8) that measures flow rates, pressures, temperatures, relative humidities of the process air and the supply air, a flow rate and a temperature of humidifier water, and an atmospheric pressure; and an arithmetic unit that receives the measured values from the measuring equipment and calculates an absolute humidity (x 1 [kg/kg (dry air)]) of the process air, an absolute humidity (x 2 [kg/kg (dry air)]) of the supply air, an amount of the process air, a required amount of cooled-dehumidified air, a required amount of cooling-dehumidification-byproduct water, a required amount of cooled air, and a required amount of humidification, when the absolute humidity (x 1 [kg/kg (dry air)]) of the process air is higher than the absolute humidity (x 2 [kg/kg (dry air)]) of the supply air, that is, x 1 ≥ x 2, causing the required amount of cooled-dehumidified air out of the amount of the process air to flow downstream as a main stream in a main-stream duct in which a cooling dehumidifying unit, a heater and a humidifier are mounted; and causing the remainder of the amount of the process air, that is, (the amount of process air) - (the required amount of cooled-dehumidified air), to flow downstream as a side stream in a side-stream duct in which a regulator (43, 44) is provided for regulating a flow rate of the process air in the side-stream duct. in an air conditioning system including: measuring equipment (33, 34, 35, 36, 49b, 28, 6, 8) that measures flow rates, pressures, temperatures, relative humidities of the process air and the supply air, a flow rate and a temperature of humidifier water, and an atmospheric pressure; and an arithmetic unit that receives the measured values from the measuring equipment and calculates an absolute humidity (x 1 [kg/kg (dry air)]) of the process air, an absolute humidity (x 2 [kg/kg (dry air)]) of the supply air, an amount of the process air, a required amount of cooled-dehumidified air, a required amount of cooling-dehumidification-byproduct water, a required amount of cooled air, and a required amount of humidification, measuring equipment (33, 34, 35, 36, 49b, 28, 6, 8) that measures flow rates, pressures, temperatures, relative humidities of the process air and the supply air, a flow rate and a temperature of humidifier water, and an atmospheric pressure; and an arithmetic unit that receives the measured values from the measuring equipment and calculates an absolute humidity (x 1 [kg/kg (dry air)]) of the process air, an absolute humidity (x 2 [kg/kg (dry air)]) of the supply air, an amount of the process air, a required amount of cooled-dehumidified air, a required amount of cooling-dehumidification-byproduct water, a required amount of cooled air, and a required amount of humidification, when the absolute humidity (x 1 [kg/kg (dry air)]) of the process air is higher than the absolute humidity (x 2 [kg/kg (dry air)]) of the supply air, that is, x 1 ≥ x 2, causing the required amount of cooled-dehumidified air out of the amount of the process air to flow downstream as a main stream in a main-stream duct in which a cooling dehumidifying unit, a heater and a humidifier are mounted; and causing the remainder of the amount of the process air, that is, (the amount of process air) - (the required amount of cooled-dehumidified air), to flow downstream as a side stream in a side-stream duct in which a regulator (43, 44) is provided for regulating a flow rate of the process air in the side-stream duct.