When no messages exist which must be notified to a mobile transmitting/receiving apparatus (102), a carrier signal of non-modulation is outputted in an area of control information of transport data having a frame structure. As a specific example, an FSK modulator (16) causes a carrier signal of non-modulation to be outputted by a transmitting circuit (17) from a line control apparatus (101) in correspondence with the area for control information in which non-modulation data is set by a frame data processing unit (15), while causing a modulated wave signal as four-level FSK modulated to be outputted in the areas other than the non-modulation data area. When non-modulation data is set in the area for control information, function channel identification information in the same frame is set as non-modulation identification data, thereby allowing any unstable operation in the mobile transmitting/receiving apparatus (102) on the receiving side to be avoided.