When a data transfer request is received, a transmitting-side NIC generates an RDMA packet from transfer data and speculatively transmits it without inquiring the transfer destination about presence of reception permission. If reception of a receiving area is unpermitted, the RDMA packet is retransmitted when a retransmission request is received from the transfer destination. When transfer unpermission is determined by referencing transfer area control information upon packet reception, a receiving-side NIC discards the received packet and, if transfer permission is determined thereafter, transmits a retransmission request, thereby transferring it. If reception is unpermitted, that may be notified to the transmission source, thereby ceasing the transfer.