Among a plurality of microprocessors 12, 32, when the load on a microprocessor 12 which performs I/O task processing of received I/O requests is equal to or greater than a first load, the microprocessor assigns at least an I/O task portion of the I/O task processing to another microprocessor 12 or 32, and the other microprocessor 12 or 32 executes at least the I/O task portion. The I/O task portion is a task processing portion comprising cache control processing, comprising the securing in cache memory 20 of a cache area, which is one area in cache memory 20, for storage of data.