To facilitate the opening and closing of a gate door and to surely hold such a holding state when it is closed.

A gate door 1 is mounted to a post 3 through opening and closing devices 4 and 5, and the opening and closing devices 4 and 5 are constituted of a base member and a pivotal member pivoted on the base member each other horizontally in a rotatable manner, a fitting section capable of fitting the rear section of the base member thereto is formed in the pivotal member, the pivotal member is fixed to one side of the gate door 1, the base member is fixed to the post 3, an oblong shaft hole inserting a bearing thereinto is formed in the base member, an elastic member having one end coming into contact with the bearing and the other end coming into contact with the end of the shaft hole is stored in the shaft hole, the pivotal member is always energized in the direction of one side by elastic force thereof, the rear section of the base member is fitted to the fitting section proided to the pivotal member when the pivotal member is positioned sideways by the elastic force and is held in a closing state of the gate door 1, the pivotal member together with the gate door 1 is moved to the direction of the other side within a range of the shaft hole against the elastic force of the elastic member by pushing the gate door 1 downward, and the gate door 1 is so constituted that the rear section of the base member is separated from the fitting section to rotate the gate door 1 in the vertical direction to open.