PROBLEM TO BE SOLVED: To provide a transverse flow sedimentation basin in which a flow straightener in which the maintenance is useless and the carry in and carry out are easy is disposed in the transverse flow sedimentation basin in which the ceiling is lidded.SOLUTION: The transverse flow sedimentation basin is characterized in that two or more water stream inclination plates are spread to oppose water stream providing an interval back and forth in a rectification sedimentation tank in the transverse flow sedimentation basin in which a ceiling is lidded.