A modular data center includes a fan creating a first cooling wind from outside air, an air conditioner creating a second cooling wind having a temperature lower than that of the outside air, racks housing electronic devices that take in the first and the second cooling wind, and a control unit adjusting an air volume of the first cooling wind by controlling the fan, and so as to cool the electronic device to a specified temperature. The control unit executes stopping the air conditioner when a first assumed value of a sum of air-conditioning power of the air conditioner and the fan is smaller than a current value of the air-conditioning power, and operating the air conditioner when a second assumed value of the sum of air-conditioning power of the air conditioner and the fan is smaller than the current value of the air-conditioning power.