A digital radio communications receiver for predicting correctly a frame structure and assuring correct synchronization. The digital radio communications receiver for use in a digital communications system having two or more frame structures on a single channel, comprises a unique word detector module for detecting a unique word from a received bit string, a receive timing controller for timing controlling a received frame based on the unique word detection information from the unique word detector module, a frame structure determining module for determining the frame structure based on the unique word detection information from the unique word detector module and the frame structure determining guard level, and a frame structure determining guard level setting module for setting the frame structure determining guard level that is the number of consecutive detections of the frame structure in frame structure determination conditions and outputting the resulting guard level to the frame structure determining module, whereby the probability of erroneous detection of the frame structure is lowered by recognizing a change in the frame structure and by outputting the information about the new frame structure.