Extension terminal device, communication system, communication method, and communication program

An extension terminal device (4) according to the present disclosure comprises: a first communication unit (41) that performs communication with a terminal device by using a first communication method; a second communication unit (42) that performs communication by using a second communication method different from the first communication method; a multi-hop communication control unit (44) that performs control of multi-hop communication in a multi-hop network where an aggregation device serves as a master station; and a protocol conversion processing unit (43) that performs protocol conversion of uplink data, which is data to be sent to the aggregation device from the terminal device, from the first communication method to the second communication method when the first communication unit (41) receives the uplink data. The second communication unit (42) transmits, to a counterpart device that is the next node on an uplink path to the aggregation device, the uplink data that has undergone the protocol conversi