An underwater sounding apparatus includes a transmitter section (3) for transmitting an acoustic signal in a specific direction underwater from a ship, a receiver section (4) for receiving an echo reflected from inside a fish school FS as a result of the aforementioned transmitted acoustic signal by means of a receiving beam, and a signal processing section (5) for processing signals picked up by the aforementioned receiving beam. The signal processing section (5) calculates fish quantity information about the aforementioned fish school FS by integrating equivalent input sound intensities obtained by the aforementioned receiving beam in specific three-dimensional directions.