Transmitting device, receiving device, packet transfer system,packet transfer method, and packet transfer program

A transmission device (10) includes a flow table (11) that stores identification information about an uninterruptible target flow; a transmission-side identification unit (12) that identifies whether a received packet is from the target flow or a non-target flow based on whether the received packet matches the identification information about the target flow stored in the flow table (11); a tag application unit (13) that applies, to packets from the target flow, an uninterruptible identifier indicating that the packets are from the target flow and a sequence number for distinguishing the packets from other packets; and a branch unit (14) that branches the packets from the target flow processed by the tag application unit (13) into packets to be transferred to an active path (41) among redundant routes and packets to be transferred to a backup path (42) among the redundant routes.