PURPOSE: To form a collision prevention alarm generating device, of simple, low cost, small and easy to be operated, which operates at always under all weather an, in addition, designed especially to be mounted on a small leisure boat or a fishing boat to detect navigation radar on a medium-size or large ship.

CONSTITUTION: Directional antennas 1-4 which detect navigation radar signal are so assigned as to evenly share sector of four-circumference of a small ship B. The signal sensed with theses antennas are detected with detectors 5-8, respectively, and then amplified with amplifiers 9 and 12, then sent to adders 17 and 20. At the adder 17, the difference between the output of antenna 1 and the total output of other three antennas is taken, and sent to a logical gate 25. Similar calculation is performed with other antenna outputs, and sent to logical gates 26-28, respectively. Each logical gate contains another input which is controlled by the output of a compratrator 31, and turns on a green or red display lamp according to a threshold value, or generates the signal which emits acoustic alarm.