To provide an intrusion preventing structure for a vessel, which can prevent intrusion of outsiders into exposed stairs.

The intrusion preventing structure 100 for a vessel including a plurality of stories of residential blocks 2 provided on a deck 11 includes exposed stairs 31, which make an access to each of the plurality of stories of the residential blocks 2 possible. The whole or one part of a part of a stairs part 3, which is provided to be exposed outside in the outside of the residential block 2, is covered with a seal part 4A, which is formed of a fixed fence and an opening/closing fence 42, to seal against the outside, and a wire 33 connected to a lower part of the stairs 31a is wound up by a winch 34 so as to house the stairs 31a in a landing 32a.