To provide opening/closing structure of an opening surely and easily opening/closing the opening, by constantly and smoothly moving a sliding door, even if form accuracy of a wall near the opening or a sliding door is not so high, and to provide high-pressure air chamber.

The opening 4 formed to the wall 5 is opened/closed by the sliding door 10 which is guided by a guide member 20 to move along the wall 5. A bag body 30 which is long in the moving direction of the sliding door 10 and in which fluid is injected, is attached to the sliding door 10 or the guide member 20. Along with change in volume of the bag body 30 by the fluid, the sliding door 10 is pressed against the wall 5 around the opening 4 through packing 6.