PURPOSE: To measure pressure at an optional internal position without invading from outside of a body by detecting a fine air bubble, produced in a measurement medium by using a low-frequency ultrasonic wave, by the transmission, scattering, and reflection of a high-frequency ultrasonic wave with which a part to be measured is irradiated.

CONSTITUTION: A fine air bubble produced in a measurement medium by using a low-frequency ultrasonic wave is detected by the transmission, scattering, and reflection of a high frequency ultrasonic wave with which a part to be measured is irradiated to perform measurement on the basis of the relation among previously known low-frequency ultrasonic wave pressure, measurement medium pressure, and air critical pressure for generating air bubbles. For example, a transducer 1 for sweep pressure generation provided with a air bubble detecting, transmitting and receiving transducer 5 in the center and a body 2 to be measured are placed in the water and a reflecting plate 6 is opposed to them in the water. Their distance is set (n) times of a half wavelength λ/2 and the transducer 1 transmits a continuous wave of wavelength λ to cause resonance with the reflecting plate 6, and thus a standing wave is generated to set the center of the oscillation loop at a measurement position 4.