A system has a data structure in which a value can be obtained from a key. In a write access, a first pair and a second pair are stored respectively in a volatile storage device. The first pair is saved in a nonvolatile storage device before returning a response, and the second pair is saved in the first storage device at any time with the second pair saved in the volatile storage device. In a read access in which a value is obtained from a key, it is determined that data is not stored normally if the second pair is not found in processing in which after obtaining the hash value of the value from the first pair, the second pair is read.