To secure flexibility of the memory extension area which is secured on the memory of the host computer and used by the memory controller.[Solution] A controller memory stores data corresponding to an area allocated to a memory in a memory controller configured to control the memory. An access control unit allocates a partial area of the controller memory to a host memory in a host computer and uses the areas as a memory extension area. The extension area managing unit performs management so that a size of the memory extension area in the host memory is changeable.