When a data storage request is issued, data is stored at an address indicated by a pointer that indicates the start of a free area, and when a data fetch request is issued, data is fetched from a data storage area indicated by an oldest index. Moreover, when a condense request is issued, after temporarily saving data from the data storage area to a save area, the saved data is restored to a contiguous area starting from an address indicated by a pointer which indicates the start of the free area, and the pointer which indicates the start of the free area is advanced by an amount equivalent to the size of the data storage area required to restore the data. Therefore even during the condense processing, data can be stored in the address indicated by the pointer indicating the start of the free area.