To provide a breakaway alarming device constituted at a low cost without any deterioration in breakaway prediction accuracy for surely performing breakaway prediction at a motorboat race.

This breakaway alarming device includes a laser radiation unit 12 installed in a predetermined position behind a starting line 1 for radiating a measuring laser beam 14, light receiving units 21, 22 arranged in a boat 20 at a predetermined interval for receiving the measuring laser beam and outputting light receiving time information, and a computing means 23 computing a time to an arrival of the boat at the starting line on the basis of the light receiving time information.