DATA REPEATER

PROBLEM TO BE SOLVED: To provide a data repeater for relaying the data transmission of a mobile station outside the service area of a base station to the base station. SOLUTION: This data repeater 1 for relaying the data transmission between the base station 3 and the mobile station 2 of a mobile communication system is provided with a first modem means 16 for demodulating radio signals received from the base station 3 and modulating the radio signals to be transmitted to the base station 3, a second modem means 11 for demodulating the radio signals received from the mobile station 2 and modulating the radio signals to be transmitted to the mobile station 2 and an electric field monitoring means 110 for detecting a radio frequency used by peripheral cells. Then, transmission and reception are performed with the mobile station 2 by using the radio channel of a frequency not detected by the electric field monitoring means. An original channel can be set for the transmission and the reception with the mobile station 2, and by selecting the radio channel of the frequency not used by the surrounding cells as the channel, data are relayed without causing interference with surroundings.