To provide an air conditioning method and its device capable of preventing fluctuation of performance of an evaporator by controlling the dryness of the evaporator to that according to a situation (magnitude of load and cycle balance) at that time.

A gas-liquid separator 5 is connected between an expansion valve 4 and the evaporator 6, a liquid refrigerant part separated by the gas-liquid separator 5 is connected to the evaporator 6, a gas refrigerant part is connected to an outlet side of the evaporator 6 through a bypass pipe 8, a variable throttle mechanism 9 for variably controlling a flow rate of a gas refrigerant flowing in the bypass pipe 8 is connected to the bypass pipe 8, a target dryness X is determined to control the evaporator 6, and the throttling amount of the variable throttle mechanism 9 is controlled to achieve the target dryness X.