A storage system receives a write request which specifies a logical volume address associated with a RAID group, and makes a first determination whether write target data in accordance with the write request exists in a cache memory. When the first determination result is negative, the storage system makes a second determination whether at least one of one or more conditions is met, the condition being that random write throughput performance is expected to increase by asynchronous de-staging processing of storing the write target data in the RAID group asynchronously to write processing performed in response to the write request. When the second determination result is negative, the storage system selects, for the write request, synchronous storage processing, which is processing of storing the write target data in the RAID group in the write processing and for which a load on a processor is lower than the asynchronous de-staging processing.