A dither clock generating section 1 generates a dither clock. A transmission pulse generating section 2a generates a transmission pulse emitted to the outside from a transmitting antenna 2c on the basis of the dither clock. A sample hold section 6 samples a reception pulse, received by a receiving antenna 3a, on the basis of a sample pulse obtained by time sweeping of the changing timing of the dither clock, so as to produce a long-period reception pulse that is obtained by expanding the reception pulse on a time axis.