A subsurface magazine has an explosive storage chamber,a horizontal passageway,and a vertical shaft for vent.It was proposed and legislated in Japan.The authors found that small amount of water on the floor of its storage chamber mitigated the blast pressure remarkably [1].The mechanism of the mitigation has been studied.In this study,the explosion experiments in a transparent,square cross section,and straight tube were carried out to examine mechanism of mitigation.The blast pressure in the tube and around the tube was measured.A high-speed camera was also used to observe inside the tube.