A central station generates and transmitted data of a transmission format. The transmission format includes an I/B bit indicating permission or inhibition of a new transmission of a next transmission timing of terminal stations, an R/N bit indicating reception or non-reception of a signal at the central station, and partial data produced from data received at the central station in accordance with a predetermined rule. One or plural of the terminal stations transmit the signal containing signal-length data W to the central station. The terminal station starts new transmission when the I/B bit indicates the permission of transmission. The central station determines and transmits the I/B bit, the R/N bit, and the partial data in accordance with the reception of the signal-length data W and the data transmitted from the terminal station. The terminal station determines whether or not a signal has been correctly received at the central station, in accordance with the R/N bit and the partial data. The terminal station suspends the transmission of signals for a predetermined period of time, when it is determined that the signal has not been correctly received at the central station.