Data conversion processing for converting access target data into data of a different block size is executed without degradation of data input/output processing performance.A first controller manages its own load by associating it with LUs, which are access targets of an access requester; and if the first controller receives an access request from the access requester, it judges whether its load exceeds a threshold or not; and if its load exceeds the threshold, the first controller executes first data conversion processing for converting data relating to an access target LU(s) of the access requester from a first block size into a second block size or second data conversion processing for converting the data from the second block size into the first block size; and if its load is equal to or less than the threshold, the first controller gives a command to a second controller to execute the first data conversion processing or the second data conversion processing and the second controller executes the first data conversion processing or the second data conversion processing based on the command from the first controller.