To eliminate the risk of collision during passage through a door by enabling a horizontally-rotary door to be only unidirectionally opened/closed from either of directions, in the opening/closing of the horizontally-rotary door.
Two locking pawls L (5) and R (6) urged to the side of the door are provided in a paired manner so as to keep the horizontally-rotatable door in a closed state; and a cam 2 (4), which is equipped with an unlocking mechanism for the locking pawl, is arranged in an intermediate portion between the locking pawls. Thus, the door can be opened only on a pull-side by a mechanism which makes an operation portion pulled in the direction of opening the door from a closed state, so that the pull-side locking pawl can be released and so that the push-side locking pawl cannot be released. In the direction of closing the door from an opened state, a cam pawl (1) is pressed against a wall, and the cams 1 (2) and 2 (4) are put into a driven state. Thereby, the door can be locked in a closed position by releasing the push-side locking pawl and making the push-side locking pawl housed and locked in a locking groove.