A positional information determining apparatus capable of reducing power consumption markedly in simple structure in radio transmission of positioning results to a navigation device. Satellite waves S1 to S4 are received from a plurality of GPS satellites 2 to 5 and analyzed at prescribed time intervals, thus obtaining positional data D5 denoting own current position. Only when positional data D5 can be determined, the positional data D5 is transmitted by radio to the navigation device that uses the positional data D5. Thus it is made possible to reduce power consumption with certainty because radio transmission is made only when positional data D5 can be obtained.