PROBLEM TO BE SOLVED: To provide a transverse flow sedimentation basin which can reduce the addition amount of a flocculant and can efficiently remove pollutant particles.SOLUTION: A transverse flow sedimentation basin has a rapid stirring pond and a slow stirring pond which have corners. In the transverse flow sedimentation basin, at least at one corner of the rapid stirring pond and the slow stirring pond, a baffle plate is provided so as to have a space between both flanks thereof and wall surfaces of the rapid stirring pond and/or the slow stirring pond, and further a high concentration flocculation hopper for making a flock at a storage part circulate to the rapid stirring pond is provided.
