SEPTIC TANK

PROBLEM TO BE SOLVED: To provide a work chamber forming structure capable of advancing work in a state more improved in workability in the work chamber forming stage in a septic tank main body by providing to a partition wall the positioning part externally coming into contact with the demarcation body, which demarcates a large work chamber into a plurality of small work chambers, to prescribe the arranging position of the demarcation body. SOLUTION: A large work chamber is partitioned by partition walls to form small work chambers at the part corresponding to an aerobic treatment tank, that is, a carrier fluidizing tank E1, a filter tank E2 and a treated water tank T1. A pair of angled molded materials 8A, 8A are fixed to the intermediate positions of the opposed partition walls 6, 6 at a constant interval and a container-shaped demarcation body 9 having the width reaching both partition walls 6, 6 is formed so as to be fitted in the space between the angled molded materials 8A, 8A. The demarcation body 9 has a rectangular cross section coming into contact with both partition walls 6, 6 and the angled molded materials 8A, 8A are externally brought into contact with four corner parts of the demarcation body 9 to form a second position regulating member 8 as the positioning part of the demarcation body 9.