PURPOSE: To observe the activity information of low-altitude atmosphere securely and excellently, by providing a Doppler radar using laser beams outputted simultaneously at different frequencies.

CONSTITUTION: Two sets of laser beam differing in frequency emitted by a laser oscillator 1 runs in the same optical path in terms of time and space at excellent coherence to be scattered by the atmosphere. The frequency displacement due to fluctuations of the atmosphere caused by the specified frequency difference is canceled in the going and returning optical paths, and the laser beam to be detected having excellent coherence and less reduction of strength which is frequency-shifted depending on the target aerosol or the like is detected by a receiving optical system 4. When these detection signals are applied to a mixer 5 of high response frequency characteristic, signals of differential frequency components corresponding to the activities of aerosol or the like are generated. These components are beated with the differential frequency components coinciding in time therewith, which is outputted from the mixer 5 of the radiation laser beam through mixer 3 and variable delay circuit 6, in a mixer 7, so that the activity information of low-altitude atmosphere which cannot be observed by transmission of electric waves or the like may be observed securely at excellent S/N ratio.