Application of ultrasound for accelerating the extraction of nutriments in food processing has been attempted. However, conditions of exposure to ultrasound were not clear in previous studies. This paper reports on the relationship between the ultrasonic pressure and the amount of extracted protein from soybeans. Experiments were conducted using a beaker, in which the ultrasonic fields were precisely measured. Soybean flakes suspended in water were put in the beaker and placed in a water tank. The amount of extracted protein in water upon ultrasonic exposure was calculated by the Kjeldahl method. It was found that the amount of extracted protein increased in proportion to ultrasonic pressure up to the total amount of soybean protein soluble in water. Furthermore, this paper describes the denaturation of the protein produced by the ultrasonic cavitation.