A communication system according to the present invention includes a plurality of ad hoc communication networks each formed by one or more communication units and one gateway and includes a monitoring server that monitors the ad hoc communication networks. The monitoring server determines on the basis of a monitoring result of load states of the gateways and a monitoring result of a congestion state of radio traffic whether it is necessary to carry out load distribution control for moving a communication unit subordinate to a first gateway to be subordinate to a second gateway set around the first gateway and, when determining that it is necessary to carry out the load distribution control, instructs at least one gateway of the first gateway and the second gateway to broadcast-deliver a signal including control information and instructing reselection of a gateway set as a participation destination.