A communications system for communications by a multicarrier transmission mode between a plurality of communications apparatuses, wherein an overall frequency band assigned to the communications system is divided into a plurality of frequency bands each having a constant bandwidth and a specific band. The divided bands are used to transmit used frequency band information thereby indicating the assignment of the remaining bands used between said communications apparatuses. The specific band is defined as a main band for transmitting control channel information including used frequency band information and data channel information. The main band may be added with an extension band for transmitting further data channel information. The main band and extension band can be changed in the frequency band used along with time or can be changed in the number thereof. Both the main band and the extension bands are preferably used for multiplexing by some of communications apparatuses.