An information storage device includes a memory and a processor coupled to the memory. The memory is configured to store therein a plurality of data. The processor is configured to receive a write request of writing first data to the memory. The processor is configured to perform an exclusive OR operation on the first data and second data of the plurality of data to obtain first difference data. The processor is configured to encode the first difference data by a run-length encoding to obtain encoded data. The processor is configured to determine whether a first size of the encoded data is smaller than a predetermined threshold value. The processor is configured to store, upon determining that the first size is smaller than the predetermined threshold value, the encoded data in the memory in association with first position information indicating a position of the second data.