According to an embodiment, a controller includes a write control unit configured to make a control that converts data requested to be written by an external device into pieces of cluster data with a size of a cluster of a storage medium, compresses each piece of cluster data, determines a corresponding physical address of a write destination in the storage medium according to a predetermined rule, and writes the compressed pieces of cluster data to the storage medium using the physical address of the write destination. The write control unit also makes a control that writes a correspondence between the physical address and a corresponding logical address to a storage unit. The controller also includes a read control unit configured to a control that reads a piece of cluster data from the storage medium using an acquired physical address, and decompresses the read piece of cluster data.