PURPOSE: To calculate the true internal pressure by measuring the pressure in a pressure vessel of which the pressure fluctuates periodically by means of two pipes having different lengths and comparing the phase difference of pressure waveforms and the difference in frequency components.

CONSTITUTION: The columnar oscillations generated in pipes 2, 3 differe in frequency and magnitude thereof because the pipes 2, 3 vary in length. Therefore, if the frequency components having differences in amplitude are removed, the pressure components of only the internal pressure of a pressure vessel 1 can be extracted. More specifically, the pressure signals detected with pressure gages 4, 5 are inputted as digital signals via an amplifier 7, a scanner 8 and an A/D converter 9 to a central control unit 10. In the unit 10, the data of the two crown angles θ-pressure P is once stored in a disc 11, and is operated, whereby the curve of the pressure and the crown angle is obtained. This curve provides the pressure waveforms of only the internal pressure of the vessel 1 wherein the oscillations in the pipes are cut off. The phase of the curve is deviated and the true internal pressure of the vessel 1 is calculated.