A method of controlling a counter-rotating axial-flow fan, the counter-rotating axial-flow fan comprising: a housing including an air channel having a suction port at one end thereof and a discharge port at the other end thereof; a first impeller including a plurality of first blades that rotates in the air channel in the vicinity of the suction port; a first motor for rotating the first impeller around an axial line; a second impeller including a plurality of second blades that rotates in the air channel in the vicinity of the discharge port; and a second motor for rotating the second impeller counter to the first impeller around the axial line, the method controlling the rotational speed of the first motor and the rotational speed of the second motor such that the temperature of an object to be cooled may be equal to or lower than a given reference temperature (Tr), characterized in that the method comprises the steps of: starting the first and second motors (11,19), which have not been driven, according to a given starting mode; controlling the rotational speed of the first motor and the rotational speed of the second motor according to a predetermined cooling efficiency mode, which gives priority to cooling the object to be cooled over reducing power consumption of the counter-rotating axial-flow fan, if the temperature of the object to be cooled is higher than the given reference temperature (Tr); and controlling the rotational speed of the first motor and the rotational speed of the second motor according to a predetermined operating efficiency mode, which maintains the object to be cooled at a temperature equal to or lower than the given reference temperature (Tr) and reduces the power consumption of the counter-rotating axial-flow fan, if the temperature of the object to be cooled is equal to or lower than the given reference temperature (Tr). a housing including an air channel having a suction port at one end thereof and a discharge port at the other end thereof; a first impeller including a plurality of first blades that rotates in the air channel in the vicinity of the suction port; a first motor for rotating the first impeller around an axial line; a second impeller including a plurality of second blades that rotates in the air channel in the vicinity of the discharge port; and a second motor for rotating the second impeller counter to the first impeller around the axial line, the method controlling the rotational speed of the first motor and the rotational speed of the second motor such that the temperature of an object to be cooled may be equal to or lower than a given reference temperature (Tr), characterized in that the method comprises the steps of: starting the first and second motors (11,19), which have not been driven, according to a given starting mode; controlling the rotational speed of the first motor and the rotational speed of the second motor according to a predetermined cooling efficiency mode, which gives priority to cooling the object to be cooled over reducing power consumption of the counter-rotating axial-flow fan, if the temperature of the object to be cooled is higher than the given reference temperature (Tr); and controlling the rotational speed of the first motor and the rotational speed of the second motor according to a predetermined operating efficiency mode, which maintains the object to be cooled at a temperature equal to or lower than the given reference temperature (Tr) and reduces the power consumption of the counter-rotating axial-flow fan, if the temperature of the object to be cooled is equal to or lower than the given reference temperature (Tr). starting the first and second motors (11,19), which have not been driven, according to a given starting mode; controlling the rotational speed of the first motor and the rotational speed of the second motor according to a predetermined cooling efficiency mode, which gives priority to cooling the object to be cooled over reducing power consumption of the counter-rotating axial-flow fan, if the temperature of the object to be cooled is higher than the given reference temperature (Tr); and controlling the rotational speed of the first motor and the rotational speed of the second motor according to a predetermined operating efficiency mode, which maintains the object to be cooled at a temperature equal to or lower than the given reference temperature (Tr) and reduces the power consumption of the counter-rotating axial-flow fan, if the temperature of the object to be cooled is equal to or lower than the given reference temperature (Tr).