The objective of the present study is to develop a procedure to predict the cavitation erosion intensity. To this end, it is necessary to presume impact force spectra caused by collapse of cavity bubbles. In this paper the authors measured the impact force spectra on a two-dimensional foil by piezoelectric pickups. Following conclusions have been obtained : 1. It is desired to develop smaller and more reliable sensors and calibration procedure. 2. Accumulative impact force spectra are inversely proportional to impact force to power n. Where the value of n is about 2 if impact force is low. The n increases with the impact force and attains around 4 in the region of high impact force. 3. Cavitation impact intensity is considered a good index of the erosion when appropriate threshold level is choosed.