The window covering the recess of the ADCP (Acoustic Doppler Current Profiler) equipped at the bottom of a large ocean research vessel “Mirai” was broken sometimes. This paper reports about the investigated results of the reason of this breaking mainly by carrying out model experiments and by theoretical calculation of ship motions in regular and irregular waves. Some caution about the consideration of pressure distribution were found and some prevention methods were proposed. Those results will be some help to the similar situations.