Abstract: An underwater detection system which searches the water in a wide range of directions and displays moving targets in different colors depending on speeds of the targets comprises an ultrasonic transmitting unit (101) for radiating a search pulse having a carrier frequency in a wide range of directions, an ultrasonic receiving unit (1p, 1q) for successively searching areas in mutually different directions in the wide range of directions to receive ultrasonic signals coming from each of respective directions therein at least two times, a phase difference detector (10, 12), detecting a phase difference between the carriers of the reception signals having come from the same direction in the wide range of directions and caught by the ultrasonic receiving unit, a calculator (18 calculating the angular frequency of the reception signals based on the phase difference of the carriers of signals, a color signal generator (103), producing predetermined color signals corresponding to angular frequencies based on signals