An oil pump was fabricated with a transparent cover made of acryl resin in order to observe the inside of the pump. The images of cavitation in that pump were taken with a high speed camera. As a result, the behavior of cavitation from generating to collapse was revealed and the site of cavitation collapsed was agreed with the site of erosion occurred． The timing of collapse and the influence of flow were observed and it was possible to describe the process leading to erosion as a trochoid oil pump.