PURPOSE: To reduce the synchronization acquisition time by utilizing positively a synonym phenomenon caused when an inverse spread signal obtained resulting from applying inverse spread processing to a spread spectrum signal is digitized by a down-sampling method.

CONSTITUTION: When an inverse spread signal is digitized by a downsampling method in a spread spectrum receiver, a high frequency component is looped back and restores to a frequency within a frequency band less than a half of the sampling frequency. This is referred to as a synonym phenomenon. Thus, a noise signal is eliminated by a synchronizing signal detection filter 12 from the digitized signal in this way. The processing for synchronization establishment is implemented and when the synchronization is not established, the frequency search is tried for a frequency range below a half the sampling frequency and the synchronization is established. When the band width of the filter 12 is variable, the band width is made wide before the synchronization establishment to allow the synchronization to be easily established and the band width is made narrow after the synchronization establishment to a degree that an inverse spread signal is able to be passed. As the band width of the filter 12 gets narrower, the S/N of the inverse spread signal is improved.