We research energy saving technology used to cool the servers and storages at data centers, which generate a markedly high amount of heat. This study examines how to reduce energy consumption in data centers. At first we explain three kinds of methods of energy saving, which are (i) energy saving of IT devices, (ii) energy saving of electrical supply systems, (iii) energy saving of air conditioning systems. After then we explain integrated management systems between IT devices and air conditioning systems. Therefore, we save more energy than stand alone systems.