Abstract: In a network system wherein a plurality of terminal devices communicates with each other via respective node devices over a ring transmission line, a transmitting node device can confirm whether a multicast information transmission has succeeded or failed. The transmitting node device send a response frame after a multicast information frame. A receiving node device relays the multicast information frame and a response frame from the upstream node device to the downstream one when the multicast information has been received successfully, or in case of a failure in receiving the multicast information, sends a response frame to the downstream node device by changing at least part of the response frame from the upstream node device. The transmitting node device can determine from a received response frame if there is one or more of the receiving node devices which cannot receive the multicast information.