A sensitive apparatus for measuring the thickness of dynamic liquid film under various conditions is being presented. The fluctuation curve of film thickness can be determined continuously by employing this apparatus. The film thickness is proportional to the height of its amplitude generated on the fluctuation curve. This would provide simpler calibration and more accurate measurement. The error is less than 2.2% within the measurement range of around 1 mm. A study on annular gas-liquid two-phase flow

using this apparatus as a measuring means

is being chosen as an example to prove its validity.