To provide a septic tank having sufficient strength to force from a side surface acting in parallel to a flange surface of a connecting part and force acting in the separating direction of an adhesive layer of the flange surface.

In a septic tank 10 where a vessel main body 11 whose one surface is opened and which has flanges 12 overhanging outside from the edge periphery of this opening is manufactured, and upper surfaces of the flanges 12 and 12 of this vessel body 11 are superposed on each other so as to be opposed to each other, and the flanges 12 and 12 are integrally connected to each other, projecting parts 12 and recessed parts 13 having a length almost equal to these projecting parts 12 are arranged on upper surfaces of the flanges 12 of the vessel body 11. When the upper surfaces of the flanges 12 of the vessel body 11 are connected to each other by superposing them on each other, the projecting parts 12 and the recessed parts 13 are fitted to each other.