PURPOSE: To secure watertightness and permit the opening and closing of a prescribed hatch cover by installing an advancing and retreatable packing onto one edge surface of the contiguous hatch covers and a tightening plate which can be advanced and retreated freely onto the other edge surface.

CONSTITUTION: A packing 23 is installed onto one of the edge surfaces of pontoon-type hatch covers 211 and 212 which are arranged in parallel so as to cover a hatch port, and a tightening plate 25 is installed onto the other edge surface. Each packing 23 and tightening plate 25 can be advanced and retreated through the supporting rods 27 and 28 which are operated by a driving apparatus. Therefore, sufficient water tightness can be secured by advancing the packing 23 and the tightening plate 25 and by butting them each other, and only the prescribed hatch covers can be opened and closed by allowing the packing and the tightening plate to retreat from each other.