The invention is aimed at reducing a frequency of occurrence of communication defects between a ground station and a mobile body running on a prescribed running path. The characters of achieving the purpose of first invention are following structure: a communication line (3) is prolonged through a repeater station (8) whose send receive frequency (f1, f2) between a lateral communication line (3A)and a mobile station (5) and whose send receive frequency (f3,f4) between a secondary lateral communication line (3B) and a mobile station (5)are set to be different from each other. The frequency of the mobile radio signals and the ground station signals are changed to set to the frequency of a downstream side of the communication line (3A, 3B) through the direction of these signals, the send receive frequency of the mobile station (5) is switched to set to the send receive frequency of the downstream side of the communication line (3A, 3B) while passing the repeat station (8).