A refresh controller transmits two refresh request signals of a first request signal which indicates a time at which a refresh operation of a DRAM may be performed and a second request signal which indicates a time at which a refresh operation of the DRAM must be performed, to an arbitrator. On the other hand, also transfer request signals each of which requests a data transfer are transmitted from plural data transfer parts, respectively, to the arbitrator. If no transfer request signal is input when a first request signal is input to the arbitrator, a refresh operation of the DRAM is performed. As a result, a refresh operation is performed when the crowding level of a bus is relatively low. This improves an efficiency in a data transfer.