PURPOSE: To ensure the safety of a work by injecting a fluid into a ring-like bag which is moored at the inner wall of a manhole normally in its contracted state to be expanded at the time of a manhole work, thereby surrounding the opening part of the manhole.

CONSTITUTION: At the time of a work in a manhole 3, a ring-like bag 6 which is stored in its contracted state in the manhole 3 is taken out from the opening part 4 of the manhole onto the road surface 5, and a fluid such as air, water or the like is injected into the bag. Whereupon, the ring-like bag 6 is expanded in such a manner as to have a height required for protection and such an inside diameter as to enclose the manhole opening part 4, kept in shape like a fence, and placed in such a manner as to enclose the opening part 4. At this time, an entrance 11 for permitting a worker to go in and out is formed on a part of the side wall. Subsequently, after the end of the manhole 3 work, the fluid in the bag 6 is discharged to contract the bag, and the bag is moored at the inner wall of the manhole 3 through a fitting member 8 having a bending part comprising plural hinges 10,10 having a pivot in the horizontal direction.