A processor is configured to determine, if a beginning or an end of a target storage area into which write data is to be written is included in a first storage area in a first storage unit, whether the beginning or the end of the target storage area coincides with a beginning or an end of the first storage area. The processor is configured to extract, if it is determined that the beginning or the end of the target storage area does not coincide with the beginning or the end of the first storage area, data identical to data stored in a sub-storage area in the first storage area from a second storage unit. The sub-storage area is not overlapped with the target storage area. The processor is configured to generate data to be written into the first storage unit by using the extracted data and the write data.