In response to a write request from a master to write to an external device, a control unit holds a write address and write data from the master in a write address holding unit and in a write data holding unit, respectively, outputs a reception signal to the master, and writes the write data to the external device specified by the write address. When the master holds the read address in the read address holding unit, the control unit reads data from the external device specified by the read address, and holds the read data in the read data holding unit.