The present invention provides a movable breakwater that can do with a small amount of compressed air supplied to raise the buoyant steel pipe. The buoyant steel pipe 6 includes a buoyancy tank 6d in which gas is filled, an air chamber 6e provided above the buoyancy tank 6d, a penetration pipe 19 that penetrates through the buoyancy tank 6d and feeds compressed air supplied from a pressure accumulator tank 13 to the air chamber 6e, and an open chamber 6f, whose top side is open, provided above the air chamber 6e and in the buoyant steel pipe 6 at the upper end portion thereof.