To conduct work readily even in an area limited for the height of a working case by surrounding an object to be worked constructed in water with a lower case body on the level, sinking the working case formed by joining the upper case body on the upper face of the lower case body, draining the inside surrounded by the working case, and thereby forming a dry working space.

A working case 1 is formed of an annular lower case 2 consisting of a pair of semicircular assembly units 4 fabricated and an upper case body 3 formed in three-layered relation of a case bodies 12 having six steel segments 11 joined with each other. An object to be worked constructed in water is surrounded by the lower case 2 on the level, and the upper case body 3 is fabricated on the upper face for thereof forming a working case 1. By pouring water in the ballast control tank 7 of the lower case body 2. the working case 1 is caused to sink and the bottom part of the working case 1 is sealed, and thereafter the water in the inner part is drained by a pump in order to define a working space. Since the working case 1 can be towed undergoing no limitation of the height of an object to be worked or depth of water, a dry working space can be formed readily.