The trapped water on the deck affects the transverse stability of purse seiners running in stern quartering seas. In this paper, we conducted free-running model experiments in regular and short-crested irregular following and quartering waves to clarify the state of trapped water on the deck and its effect on transverse stability. In the model experiments, pressure sensors, and cameras are installed on the weather deck of the ship model to measure the state of trapped water on the deck. The influence of trapped water on the deck is quantitatively evaluated by conducting a phase difference analysis of the measured trapped water on the deck motion and the time series of the ship model's rolling motion. The results confirm that trapped water on the deck acts as a hydrostatic heeling moment in the range around the design speed.