A cache managing unit creates a list of elements corresponding to each data block arranged based on a priority of writing data blocks to a magnetic disk apparatus, and when a group of elements corresponding to data blocks to be written to the same magnetic disk apparatus exists, provides a link connecting elements at both ends of the group. A write control unit searches, upon selecting a data block for writing, elements belonging to the list in descending order of priority, and if a link is set at an element corresponding to a data block to be written to a magnetic disk that cannot perform a writing, follows the link to search a subsequent element.