PURPOSE: To eliminate hindrance from works, etc. even in case where the groove width is enlarged and the displacement capacity is improved in such a way that flange parts to cover displacement grooves are formed around the rims of hatch covers, and the rims of the flange parts are supported by a deck member.

CONSTITUTION: Hatch coamings 8 are erected around hatches 7, and displacement grooves 9 are formed around the outer peripheries of the hatch coamings 8. The displacement grooves 9 are covered by flange parts 15 formed on hatch covers 12. And the rims of the flange parts 15 are supported by supporting stepped parts 16 which are formed on a deck member 2a. By doing as this, even if the edge portions of the flange parts 15 are stepped on, other edge portions do in no case come off. Besides, as the hatch covers 12 are flush with the upper surface of the deck 2, they do not disturb works, etc. on the deck 2.