In a mobile communication system of the present invention, a cordless base unit LBU is connected to a wired subscriber line SL. The base unit LBU detects a radio channel assigned to a cell other than a cell where the base unit LBU is located, among radio channels assigned to the cells of a cellular radio communication system. This radio channel is used as a radio channel for a cordless call. On the other hand, a cellular cordless telephone CCT is provided with a mode switch. When a cordless mode is designated by the mode switch, a radio channel for a cordless call is detected, as with the case of the base unit LBU. At the time of the cordless call, a transmission power level of the cellular cordless telephone CCT is set at a weak level smaller than that for a cellular call.