The circular plastic window (we call it as acoustic window) at the bottom of a large ocean observation ship was broken sometimes. This window is a cover of the downward hole of the recess of the ADCP (Acoustic Doppler Current Profiler) of this ship. This paper report about the reason of the damage of this plastic window through the simple model tests and simple theoretical calculations. Through this considerations, the importance of the concept of the pressure by the incompressible fluid in acceleration motion is stressed.