PURPOSE: To enable the working property easy, by means of enabling the inserting and withdrawing of a shielding substance contained in the space part by setting up opening parts at the upper surface and the lower surface of a hatch, and of lightening the hatch when it is in mounting and in treanfering.

CONSTITUTION: Opening parts 12, 13 are set up at the upper surface and the lower surface of a hatch 1 which is made of iron plate 3 and is provided with a space 11 in its inner part, and a disk plate 5 is mounted on the opening part 12, and an exit plate 7 is mounted on the opening part 13 freely from opening and closing. The hatch 1 is installed on the surface of an ceiling, and the shielding substance 4 is inserted into the space part 11 from an inserting inlet 12, and the disk plate 5 is mounted. After the hatch 1 being lightened by means of taking out the shielding substance from the taking out port 13 when the hatch 1 is in dismounting or in transfering, the taking out can be effected, thereby the working becomes easy.