A method of controlling air conditioning to cool servers, the method includes selecting, when the number of operating servers is a certain threshold number or more, a first mode, in which the pressure of a cold aisle is set higher than the pressure of a hot aisle; selecting, when the number of operating servers is less than the certain threshold number, a second mode, in which the pressure of the cold aisle is set equal to the pressure of the hot aisle; and informing the operating servers of whether a current mode is the first mode or the second mode, and when the current mode is the first mode, driving fans of the servers at a minimum rotational speed in a specification, and when the current mode is the second mode, driving the fans of the servers at a rotational speed higher than the minimum rotational speed in the specification.