A storage system is provided with a plurality of nonvolatile semiconductor storage devices (hereafter referred to as semiconductor storage devices) and a storage controller that is coupled to the plurality of semiconductor storage devices and that provides an LU (logical unit) to an upper level apparatus. Each of the semiconductor storage devices is provided with a nonvolatile semiconductor storage medium (hereafter referred to as a semiconductor medium) and a medium controller that is a controller that is coupled to the semiconductor medium. In the case in which the medium controller receives a write command and a data unit from a storage controller, the medium controller writes the data unit to a physical storage region of a write destination of the semiconductor medium in accordance with the write command and updates the real write data amount information that is used for specifying a real write data amount that is a total amount of a data unit that is written to the semiconductor medium based on an amount of a data unit that has been actually written. The medium controller notifies the storage controller of the real write data amount information on a regular basis or on an irregular basis. The storage controller calculates a real write data amount of the LU based on the real write data amount information from each of the semiconductor storage devices.