A controlled device (12) sends a signal to a intermediary server (21) at a predetermined time interval, and the intermediary server (21) receives the signal from the controlled device (12) with a private address allocated by a router (11) and changes an operation mode from a passive mode to an active mode. The operation of the controlled device (12) is changed to the passive mode. The intermediary server (21) continuously establishes a communication path with the controlled device (12). The intermediary server (21), which receives control information from a control terminal (41), relays data transmitted via two communication paths, that is, a communication path established with the control terminal (41) and a communication path established with the controlled device (12), to allow the control terminal (41) to control the controlled device (12) operating in the passive mode.