To enable formation of work space in a simple method by surrounding a work target constructed underwater, sinkwise installing a work box along a water shielding guide erected at a bottom part, and perform shielding between the bottom part and the guide, and drain the water contained in the inside.

A water shielding guide 16 is constructed annularly on a dredged footing 13a such that a concrete bridge leg part (work target) 13 constructed underwater is surrounded by the guide 16. A work box or vessel 1 made of a pair of assembly unit is towed in a floating manner on the surface of water by a working boat and assembled so that the leg part 13 is positioned at the center of the work vessel 1, then water is charged into the float tank of each assembly unit, and the units are sunk on the placed cocrete 19 at the inside of the guide 16. A gap between the outer circumference of lower part of work vessel 1 and the inside of guide 16 is made watertight by swelling a water shield jig 7 with compressed air and pressing the same to the inside of the guide 16. Water contained in the vessel 1 is drained from the inside by a drain pump 23, thereby forming the same dry work space as that on the ground.