To provide a waste treatment system enabling multi-purpose constitution by some fundamental parts.

A septic tank structure has first and second lower tank members 11, 12 and the cover member 13 supported on the upper ends of these tank members. Each of the tank members has a closed lower end 14, an opened upper end 15 and the sidewall between both lower and upper ends to constituted a waste container. The tank members have flat side surfaces together arranged at a time of assembling. The cover member has a lower peripheral edge part, an manhole opening 34 and the steeply inclined wall part 35 between them and the lower peripheral edge part 33 of the cover member 13 has an oval shape corresponding to the shape formed by the upper ends of the assembled tank members. The sidewalls of the lower tank members are upwardly tapered from the lower ends thereof to the upper ends thereof and the tank members are easily fitted to each other in a telescopic state at a time of transport and housing. A plurality of thick-walled flat regions 26-29 are formed to the upper parts of the sidewalls of the tank members to connect a plurality of tank structures in various desired constitutions. The lower tank members and the cover member are produced from a polydicylopentadiene thermosetting resin material.