A first and a second terminal carry out radio communications, the first terminal selects, based on an expectation saved in the concerned first terminal, a relay terminal relaying data to be transmitted to the second terminal and transmits the data, with a first information item indicating the address of the first terminal and a second information item indicating the address of the second terminal, to the relay terminal; and the relay terminal transmits the received first and second information items, and the data, to the second terminal; and if the second terminal receives the first and second information items, and the data, it transmits a completion report to the first terminal, based on expectation values saved by the second terminal concerned, making it capable of checking whether it has been possible to surely send and deliver the data.