The blast-mitigation effects of periodic obstacles installed on the floor of a tube are examined, considering their potential application for underground/subsurface explosive storage. A straight square tube was prepared, measuring 330 mm in length, 30 mm in width, and 30 mm in height. One end of the tube was closed, and a specially designed small detonator, which contained 100 mg lead azide, was ignited near this closed end of the tube. Rectangle obstacles, fixed on a thin plate, were installed on the tube floor, and their height was varied. The pressure histories were measured at seven points on the extended centerline outside the tube. The scaled distances of these seven points from the tube exit were 4.3 m/kg1/3 to 30.2 m/kg1/3. The relation between “Without obstacle equivalency” and the opening ratio at the obstacle was obtained. The relation suggested that there might be at least two mechanisms of blast mitigation by obstacles. The opening ratio of around 20% was found to cause the equivalency of less than 10%.