To provide an aeration confinement arrangement which can prevent air from mixing into a liquid flowing through a channel which causes vertical head drop by a simple configuration.

An aeration confinement arrangement 10, which is arranged between a connection 32 of tributary waterway 30 and the inner wall 44b of a water falling passage 40 facing the connection 32 of the tributary waterway 30, comprises: a guide 12 guiding a water flowing out of the connection 32 so that it might flow down while going along a surface 13, and go along the inner wall 44b of the water falling passage 40 in a lower end 18; a hinge 16 supporting an upper end 14 of the guide 12 so that the guide 12 might rotate by setting a direction roughly perpendicular and horizontal to water out flowing direction flowing out from the connection 32 of the tributary waterway 30 as a rotation axis; and a spring 18 biasing the lower end 15 of the guide 12 in a direction adjacent to the inner wall 44b of opposite side against a side to which the tributary waterway 30 of the water passage 40 is connected.

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