The invention provides a storage system which can estimate a necessary capacity at the time of acquiring a snap shot copy without actually sampling the snap shot copy, and a control method of the same. In a storage system, a disk control unit monitors a write instruction from a host machine for a fixed period, and calculates a capacity necessary at the time of a snap shot, on the basis of the monitored information for the fixed period, in the case of receiving an instruction of calculating the capacity necessary at the time of acquiring the snap shot which makes it possible to refer to the data at a certain time point within a disk unit.