In radio communication with non-modulated signal at head of receive signal, the non-modulated signal or the head of the receive signal is detected in spite of rather large frequency error of receive signal, and frequency acquisition for coherent detection in a receiver is established during detection operation of the non-modulated signal Discrete Fourier Transform (1) is carried out for an input signal in digital form in every receive sample to provide frequency spectrum of an input signal. Then, the maximum component is selected (2) in the frequency spectrum. Next, a peak detector (3) detects peak value of the selected maximum components on time axis, and recognizes the detection of the non-modulated signal. The frequency (f.sub.e) of the non-modulated signal is estimated through interpolation of the selected component having the peak value and the adjacent component. The receive frequency is adjusted by using said estimated frequency (f.sub.e), and the adjusted frequency is subject to coherent detection.