In a user's device of a GPS, the calculation of GDOP values for determining satellites to be used for the positioning is initiated periodically with a predetermined operation period. Once they are determined in a certain operation period, the processing for the positioning is repeatedly executed on the basis of signals from the determined satellites through the operation period, until satellites to be used are determined in a succeeding operation period. Plural times of the positioning can be executed for every time of the calculation of a GDOP value. If, therefore, a frequency of executing the positioning are made almost as equal as conventionally, a frequency of carrying out the time-consuming calculation of GDOP values is decreased, whereby the load of a microprocessor in the user's device can be remarkably reduced.