A mobile communication system 1 according to the present invention comprises base station 30 and mobile station 10. The base station 30 sends a pilot symbol sequence known to the mobile station 10, to the mobile station 10, using a plurality of carriers for downward channels, and performs communication with the mobile station 10 while multiplying the channels by frequencywise same scrambling codes every symbol period. The mobile station 10 is provided with a peak detector 17 for calculating cross correlation allowing for a phase difference of the scrambling codes, between frequencywise pilot symbol sequences received from the base station 30, thereby detecting a radio frame boundary.