To switch the primary modulation system, the encoding rate for error correction code addition, or the like by estimating the propagation condition in accordance with the level of a non-correlation signal appearing in the non- correlation period of a signal obtained by detection of correlator output.

The level of the non-correlation signal included in the signal obtained by detection of correlator output is detected by a non-correlation signal level detection part 8. This detected level is compared with a preliminarily determined reference value 10 by a comparison part 9. A primary modulation part 13 is so set that it normally selects modulation based on first phase shift keying(PSK); and if the level of the non-correlation signal exceeds the reference level in the comparison part, this modulation is switched to modulation based on second phase shift keying where the number of phases is made smaller than that of first phase shift keying. In another method, the encoding rate in an error correction code addition part 12 is changed if the level of the non- correlation signal exceeds the reference level.