PURPOSE: To reduce an effect of lobbing sharply and to improve the precision in calculation of distance by calculating the distance to a target on the basis of the average value of a reception signal power at each prescribed number of hits and a known value of an effective radiant power.

CONSTITUTION: A series circuit of an integrating circuit 10 and an average reception power calculating circuit 11 is interposed between a reception peak power measuring circuit 8 and a distance calculating circuit 9, and pulse repetition interval (PRI) data are outputted from a data processing system 6 to the calculating circuit 11. In other words, a reception peak power at each hit obtained by the reception peak power measuring circuit 8 is integrated by the integrating circuit 10. The result of integration is subjected to an average value processing in the following calculating circuit 11 by using the PRI data thereby an effect of lobbing is removed. Consequently, highly-precise calculation of distance can be executed in the distance calculating circuit 9 on the basis of effective radiant power (ERP) data and a average reception power.