A data relay device (20, 23, 30, 33, 40) configured to relay a message transmitted from a source node from a first network to a second network in a middle of a passage through which the message is transferred to a destination node in a network system constructed with a plurality of kinds of networks (NW1, NW2) having different protocols, the data relay device (20, 23, 30, 33, 40) comprising: a first port to which the first network is connected; a second port to which the second network is connected; and a relay section configured to convert a first frame received from a first node on the first network through the first port into a data format pursuant to the protocol of the second network, and to transmit a post-conversion second frame to a second node on the second network through the second port, characterized in that the first frame comprises a data structure in which a first header portion pursuant to the protocol of the first network is added to a data portion comprising common routing information and the message, the common routing information comprises route information identifying all nodes through which the message is routed from the source node to the destination node, and the relay section recognizes that a next destination is the second node on the second network by referring to the common routing information acquired from the data portion of the first frame, generates a second header portion pursuant to the protocol of the second network, and replaces information identifying the second node that is of a next destination in the route information comprised in the common routing information with information identifying an own node that is of a return address, and generates the second frame by adding the generated second header portion to the data portion comprising the common routing information and the message. a first port to which the first network is connected; a second port to which the second network is connected; and a relay section configured to convert a first frame received from a first node on the first network through the first port into a data format pursuant to the protocol of the second network, and to transmit a post-conversion second frame to a second node on the second network through the second port, characterized in that the first frame comprises a data structure in which a first header portion pursuant to the protocol of the first network is added to a data portion comprising common routing information and the message, the common routing information comprises route information identifying all nodes through which the message is routed from the source node to the destination node, and the relay section recognizes that a next destination is the second node on the second network by referring to the common routing information acquired from the data portion of the first frame, generates a second header portion pursuant to the protocol of the second network, and replaces information identifying the second node that is of a next destination in the route information comprised in the common routing information with information identifying an own node that is of a return address, and generates the second frame by adding the generated second header portion to the data portion comprising the common routing information and the message. recognizes that a next destination is the second node on the second network by referring to the common routing information acquired from the data portion of the first frame, generates a second header portion pursuant to the protocol of the second network, and replaces information identifying the second node that is of a next destination in the route information comprised in the common routing information with information identifying an own node that is of a return address, and generates the second frame by adding the generated second header portion to the data portion comprising the common routing information and the message.