From a transmitter (18) and a transmission antenna (10), a first frequency signal having a fixed frequency is transmitted for a predetermined time or more, a second frequency signal having a certain frequency difference from the first frequency signal is transmitted for a predetermined time or more, and a third frequency signal having a frequency difference twice the frequency distance from the first frequency signal is transmitted for a predetermined time or more. Reflected waves from objects under measurement at the respective transmission frequencies are supplied to a reception antenna (11), a mixer (12), an analog circuit unit (13), an A/D converter (14), an FFT (15) and a signal processing unit (16). Then, the Doppler frequencies are measured for the reflected waves, objects under measurement are separated and detected at each Doppler frequency, and objects under measurements having the same Doppler frequency are separated to identify a plurality of objects under measurement from phase information and amplitude information of received signals acquired at each transmission frequency. In this manner, when there are a plurality of objects under measurement which are substantially equal in relative speed, these can be separated from each other and detected.