PURPOSE: To measure an object located at a position where it is difficult to install an acoustic transducer in ultrasonic measurement.

CONSTITUTION: An acoustic fiber 5 is connected to a piezoelectric unit which is constituted of two electrodes 1 and 2 and a piezoelectric unit which is sandwiched by them. Ultrasonic wave generated at the piezoelectric unit 3 due to application of pulses to the electrodes 1 and 2 propagates within the acoustic fiber 5 and is applied to a thin film 13 from a tip 5a for inputting and outputting acoustic wave. The reflection wave which is reflected by the thin film 13 is taken into the fiber 5 via the tip 5a for inputting and outputting acoustic wave again, is transmitted to the piezoelectric unit 3 after it propagates the inside, and is converted to an electrical signal. The obtained electrical signal is for example subjected to frequency analysis and the film thickness is obtained based on the analysis result. The tip 5a for inputting and outputting acoustic wave of the fiber 5 can also be installed at a narrow location or near an object with complex shape.