In this study, the blast-mitigation effects of periodic obstacles installed on the floor and the ceiling of a tube are examined, considering their potential application for underground/subsurface explosive storage. A straight square tube, measuring 330 mm in length, 30 mm in width, and 30 mm in height, was prepared. 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 as the test explosive. Rectangle obstacles, fixed on a thin plate, were installed on the tube floor, and their number and height were varied. The obstacles were also installed on the ceiling and the floor of the tube to examine the mitigation dependence on the number of surfaces on which the obstacles were installed. The pressure histories were measured at seven points on the extended center line outside the tube. The scaled distances of these seven points from the tube exit were 4.3 m･kg^-1/3 to 30.2 m･kg^-1/3. Our findings show that the blast-wave mitigation by the obstacles does not apparently depend on the number of obstacles. On the contrary, the obstacles on the ceiling in addition to the floor mitigated the blast wave remarkably, compared to that only on the floor, although the sum of the heights of the obstacles was identical. The obstacles whose height was more than half of the inner height of the tube mitigated the blast wave, whereas those with a height less than half of the tube height did not show any obvious mitigation effect. The number of surfaces with obstacles was found to be a dominant parameter to blast mitigation.