Abstract: A method of transmitting a large amount of data with priority for use with a CSMA/CD system includes the steps of checking to see if a transmission line has a free space in response to a data transmission request from a node serving as a source node, transmitting, if the transmission line has a free space, a priority transmission notice carried by a data-link-layer protocol signal of the CSMA/CD system from the source node to the other nodes, transmitting data for which the data transmission request has been made from the source node to a node serving as a destination node, and putting the nodes except the source node in a transmission-disabled, reception-enabled state when the priority transmission notice is received by the nodes. Also provided is an apparatus for achieving such a method.