A wireless multi-hop network of a communication system is structured with a number of the communication devices and the collecting communication device. Transmission information from each communication devices is sequentially relayed to reach the collecting communication device. Each communication device features a function for on/off control of a transmission/reception processing section. The communication device also includes: a section which appends an urgent flag to urgent information that is to be relayed and transmits the urgent information, and fixes the transmission/reception processing section to ‘on’; a section which verifies a relay by a relay destination communication device and, if verification is achieved, fixes the transmission/reception processing section to ‘on’ and switches to a state for promptly relaying received information; a section which, if the verification is not achieved, retransmits the urgent information; and a section which, when retransmitted urgent information is received, relay-transmits the information immediately.