To provide a waste water treatment system in which a pH value is kept constant by a simple constitution and the destruction of the environment is restrained.

The waste water treatment system comprises: a storage tank 1 into which waste water 3 is introduced from a gutter 2, through which the introduced waste water is made to pass and from which the treated water is discharged to the ground and which has a plurality of storage chambers A, B, C divided by partition plates 9, 12 which are arrayed from the upstream side thereof to the downstream side; a peat moss layer 5 which is arranged in the uppermost-stream storage chamber A and used for lowering the pH value of the waste water 3 that is introduced from the gutter 2 and made to pass therethrough; a communicative pipeline 8 the end openings of which are made to face respectively to the lower part 7 of the upstream-side storage chamber A and to the upper part of the adjacent downstream-side storage chamber B; another communicative pipeline 11 the end openings of which are made to face respectively to the lower part of the upstream-side storage chamber B and to the upper part of the adjacent downstream-side storage chamber C; a circulation pump 15 for circulating the treated water 13 in the downstream-side storage chamber C to the uppermost-stream storage chamber A; and a discharge pump 16 for discharging the treated water 13 in the downstream-side storage chamber C.