To provide an ad hoc network system which can prevent the amount of routing information from growing too large.

In the ad hoc network system, each of nodes 101 (101a, 101b, ...) maintains routing information which contains ID information of the next hop node 101 or a data collection terminal 102 for transmitting or forwarding messages toward the data collection terminal 102, and the number of hops to the data collection terminal 102. The data collection terminal 102 acquires the routing information maintained by each of the nodes 101, and generates and maintains aggregated routing information on the basis of the acquired routing information. For uplink transmission, each of the nodes 101 transmits or forwards a message according to its maintaining routing information. For downlink transmission, the data collection terminal 102 adds source route information generated on the basis of the aggregated routing information to a message and transmits the message.