A storage system in an embodiment of this invention comprises a virtual volume to which real storage areas are allocated depending on data amount to be stored therein and which stores a plurality of mainframe data managed in units of tracks, each of the plurality of mainframe data including control information and one or more records storing user data, and a controller The virtual volume is managed by a first real storage area storing the records in the plurality of mainframe data and a second real storage area storing the control information in the plurality of mainframe data. The controller determines not to allocate the first real storage area to the virtual volume in a case that user data in the records in the plurality of mainframe data are initial values. The control information in the plurality of mainframe data is stored in the second real storage area allocated to the virtual volume.