An object of this invention is to avoid interfering from a radio LAN transmission unit and being interfered. This invention relates a radio transmission unit for transmitting according to a second radio transmission system which transmits/receives data signal under the same frequency band as first radio transmission system in which prior to transmitting of the data signal, a transmitting station transmits a transmission request signal while a reception station receiving this RTS sends back a reception preparation completion signal. This radio transmission unit comprises CTS transmitting means for transmitting CTS according to the first radio transmission system and a duration setting circuit for setting a duration time to be overlaid on the CTS, and prior to transmitting according to the second radio transmission system, the duration time set by the duration setting circuit is overlaid on the reception preparation completion signal according to the first radio transmission system.