An industrial network apparatus which is connected to an industrial network together with a controller and another industrial device, the industrial network apparatus comprising: control data communication means configured to transmit control data in a priority transmission period that occurs periodically; and message data communication means (131, 1316A, and 1316B) configured to determine whether or not to start transmission of message data in a non-priority transmission period depending on a required transmission time of message data and remaining time of the non-priority transmission period that is different from the priority transmission period, and to transmit the message data when determining to start transmission of the message data, wherein the priority transmission period includes a first priority transmission period for transmitting control data from one of the controller and the industrial device addressed to the other, the non-priority transmission period includes a first non-priority transmission period for transmitting message data from one of the controller and the industrial device addressed to the other, the control data communication means includes first control data relay means configured to receive control data transmitted from one of the controller and the industrial device addressed to the other and transmit the control data addressed to the other in the first priority transmission period, wherein the first control data relay means starts the transmission of the control data addressed to the other before receiving the entire control data, the message data communication means includes first message data relay means configured to receive message data transmitted from the one of the controller and the industrial device addressed to the other and transmit the message data addressed to the other in the first non-priority transmission period, wherein the first message data relay means starts transmission of the message data addressed to the other after receiving the entire message data, and the first message data relay means determines whether or not to start transmission of the message data addressed to the other depending on a required transmission time of the message data and remaining time of the first non-priority transmission period. control data communication means configured to transmit control data in a priority transmission period that occurs periodically; and message data communication means (131, 1316A, and 1316B) configured to determine whether or not to start transmission of message data in a non-priority transmission period depending on a required transmission time of message data and remaining time of the non-priority transmission period that is different from the priority transmission period, and to transmit the message data when determining to start transmission of the message data, wherein the priority transmission period includes a first priority transmission period for transmitting control data from one of the controller and the industrial device addressed to the other, the non-priority transmission period includes a first non-priority transmission period for transmitting message data from one of the controller and the industrial device addressed to the other, the control data communication means includes first control data relay means configured to receive control data transmitted from one of the controller and the industrial device addressed to the other and transmit the control data addressed to the other in the first priority transmission period, wherein the first control data relay means starts the transmission of the control data addressed to the other before receiving the entire control data, the message data communication means includes first message data relay means configured to receive message data transmitted from the one of the controller and the industrial device addressed to the other and transmit the message data addressed to the other in the first non-priority transmission period, wherein the first message data relay means starts transmission of the message data addressed to the other after receiving the entire message data, and the first message data relay means determines whether or not to start transmission of the message data addressed to the other depending on a required transmission time of the message data and remaining time of the first non-priority transmission period.