PURPOSE: To prevent a stress concentration to a hatch opening as well as to facilitate the hatch cover opening and closing operation by providing a coaming having a wheel guide around the upper perimeter of the circular hatch opening, and running wheels to one of semi-circular covers which are hinged at opposing sides.

CONSTITUTION: A cargo hatch opening is made circular, and to such a circular cargo hatch opening, a circular hatch coaming 5 having a wheel guide 6 comprising a flat upper perimeter is provided. A hatch cover 8 consists of two semi- circular covers 8A, 8B which are rotatable through a hinge 9. Multiple number of wheels 11 which can run on a wheel guide surface 6 of the circular hatch coaming 5 are provided to the perimeter of one half hatch cover 8A. Compared to the prior rectangular hatch opening, owing to the circular shape, cracks due to stress concentration can be prevented, and the hatch cover opening and closing operation is facilitated.