Disclosed are a mobile station unit which can obtain azimuth information with a simple construction and a radio communication system including the mobile terminal unit. According to the present invention, in a terminal station, an arrival direction obtaining section 203 obtains an arrival direction of a received azimuth designation signal from a reference station or another terminal station using an arithmetic operation or the like. A transmitting direction forming section 204 determines the direction opposite to the arrival direction obtained by the arrival direction obtaining section 203 as a transmitting direction. A reference azimuth detecting section 207 detects the transmitting direction determined by the transmitting direction forming section 204 as a reference azimuth. An azimuth designation signal generating section 205 generates an azimuth designation signal so as to radiate radio waves having directivity in the transmitting direction determined by the transmitting direction forming section 204.