A frame synchronizing signal detecting method according to the invention is used in a data multiplexing transmitter-receiver which is provided with a transmitter-receiver that transmits and receives at least a radio-frequency signal, a modulator-demodulator that converts the radio-frequency signal to a baseband signal and vice versa and a baseband signal processor that processes the baseband signal and in which the baseband signal processor is provided with a frame synchronizing signal detector, and when a frame synchronizing signal included in received data is detected, the frame synchronizing signal detector sets the detection precision of the frame synchronizing signal before a frame synchronization link is established to a value higher than regular detection precision and sets the detection precision of the frame synchronizing signal after the frame synchronization link is established to a value lower than the regular detection precision.