Authors consider that a reflection type fiber optic probe hydrophone (FOPH) is suitable for the absolute sound pressure measurement under cavitation at high ultrasonic emission like ultrasonic cleaning machine or sonochemical reactor. The precision of the FOPH at higher sound pressure is almost same as the conventional piezoelectric type hydrophone beyond 100kHz. However, the measurement accuracy of the FOPH below 100kHz becomes lower due to cavitation bubbles. The cavitation bubbles give large refractive index variation at the probe point of FOPH. And, hence, impulse distortion appears in the output signal and the measurement accuracy becomes lower. Therefore, the sound measurement below 100kHz has not been examined fully yet. In this report, distortion components of output wave signal in FOPH under cavitation are examined. Moreover, the component caused by cavitation is found.