Authors have developed a measurement technique using Particle Image Velocimetry(PIV) for the purpose of accurate measurement of individual wave-overtopping volumes in two dimensional wind-wave flume experiments. In order to clarify the characteristics of appearance frequencies of individual wave-overtopping volumes for a vertical seawall, model experiments were conducted generating irregular waves and winds at the same time. The main conclusions are as follows: 1) Frequencies of large wave-overtopping increase as the wind becomes stronger. Probability Density Functions for normalized wave-overtopping volumes, however, are constant and agree with an exponential distribution with the expected value of 1.0. 2) Mean wave-overtopping volume can be estimated from wave overtopping rate.