A buffer is provided with a leading pointer and a following pointer. A bitmap in which two bits are assigned to each block is updated to retain which states blocks are in, busy, write-completed, or read-completed. Under the constraint that the two pointers move in the same direction and do not pass each other: after the block designated by the leading pointer starts to be written, the leading pointer is moved to a next block only if the next block is in the read-completed state; and after the block designated by the following pointer starts to be read, the following pointer is moved to a next block only if the next block is in the write-completed state.