PROBLEM TO BE SOLVED: To achieve energy saving in a reservoir tank, data center, and cooling method of the data center.SOLUTION: The data center includes: an electronic apparatus 3a cooled by cooling water; a chiller 2 for generating the cooling water; a reservoir tank 14 including a liquid tank 12 for reserving the cooling water and an outer wall 13 that is disposed around the liquid tank 12 and is opened or closed on the basis of the ambient temperature and the temperature of the cooling water; and piping 5 that interconnects the chiller 2 and electronic apparatus 3 via the reservoir tank 14 and through which the cooling water flows.