To shorten the time rate of communication interrupt and to improve quality by reducing the step-out of reproducing symbol clocks, due to the rapid degradation of a reception level and reception waveform distortion by multi-path phading.

By comparing a distribution amount, calculated over one slot length of the zero cross phase of a symbol clock component with a certain fixed threshold value in a threshold value judgement means 27, judging reception signal degradation when it is larger than that and stopping phase tracking by a digital PLL, the rapid decline of the reception level by fading and shadowing generated during communication, the waveform distortion of reception signals generated by the multi-path fading and the same channel interference by nonnegligible delay spread or transition to the step-out state of clock synchronization by the intermittent reception of a silence interval in VOX control are prevented.