A data transfer device includes a buffer unit temporarily storing transfer data to be transferred to a common bus, a write control unit writing input data as the transfer data to the buffer unit and outputs a notification signal, a read control unit reading the transfer data from the buffer unit, an interface unit transferring the transfer data to the common bus according to a predetermined bus protocol, and a band-smoothing unit which smoothes a band of the common bus by delaying the notification signal, generating a read control signal, and outputting the read control signal to the read control unit, wherein when previous transfer data which has been read from the buffer unit already exists and a DMA transfer receipt ACK responding to a DMA transfer request REQ is received by the interface unit, the read control unit reads current transfer data from the buffer unit.