PURPOSE: To make it possible to easily obtain the horizontal posture of a diversoir and to stably and exactly set this horizontal posture by pivotally supporting the supporting means of the diversoir laterally oscillatably via the fulcrum shaft in the central part of the diversoir and engaging a member for fixing the oscillation position with the diversoir.

CONSTITUTION: The water level of a treating chamber 5 rises temporarily when sewage flows into a septic tank from an inflow port and the water levels in respective treating chambers attain a specified level or above. The water from the treating chambers 5 flows over through respective notches 7a of the diversoir 7 into right and left trough parts 7f for induction as a result of such rising. The water in the trough parts 7f is introduced into a collecting trough part 7e and comes into contact with drugs. This water is fed through a conduit into a treating chamber 6 and is taken as clean water out of the treating chamber 6. The setting of the supporting mechanism of the diversoir 7 is completed by a horizontal adjusting method of merely loosening a set screw 8 and laterally oscillating the diversoir 7 around the fulcrum shaft 7b to adjust its horizontal position, then retightening the set screw 8.