Disclosed is a multicast network system including a multicast network which provides a plurality of relay nodes, and a monitoring server and a plurality of acknowledgment agents connected to the multicast network. The monitoring server includes an acknowledgment request message generating and sending unit that generates and sends an acknowledgment request message storing an acknowledgment probability to the plurality of acknowledgment agents; a collecting unit that collects acknowledgment messages from acknowledgment agents; and a collection number comparing unit that compares the number of acknowledgment messages collected by the acknowledgment message collecting unit with the expected number of acknowledgment messages obtained from the acknowledgment probability and the total number of acknowledgment agents. The acknowledgment agent has an acknowledgment message sending unit that determines whether an acknowledgment is needed for the acknowledgment request message based on the acknowledgment probability stored in the acknowledgment request message to send an acknowledgment message.