A water-inflow prevention device is provided by installing a closing lid 6 for blocking a duct 2 of an air pressure regulating valve 1, which regulates air pressure inside a cabin 101. A buoyant body 9 is attached to the disc-shaped closing lid 6 installed to the duct 2 so as to be rotatable around a rotation shaft 7. The buoyant body 9 receives a buoyant force from water when immersed in the water. Since the side of the closing lid 6 in which the buoyant body 9 is attached is thereby floated in the water, the closing lid 6 automatically rotates around the rotation shaft 7 so as to close a ventilation port 2a.