To provide a lock and door structure, by which load required for opening and closing and driving a door can be reduced and in which structure can be simplified without limiting the height direction of a ship.

The lock 1 has the first sluice gate 2 and the second sluice gate 3 being installed at a regular interval and having the same constitution. The first sluice gate 2 has a door housing section 6 installed at a specified distance in a waterway so as not to get trouble in navigating ships, a door 5 freely projected and retreated to and from the door housing section 6, and a projecting-retreating device to and from the door housing section 6 of the door 5, and the inside is hollowed and an opening is formed to an underside in the door 5, and the projecting-retreating device is composed of an exhaust mechanism discharging air in the door 5 and an air-supply mechanism 7 forwarding air into the door 5 from the opening.