The large purifying tank is composed of an upper tank portion (1) and a lower tank portion (4), or composed of an upper tank portion (1), a lower tank portion (4), and an intermediate sheet-belt tank portion connecting the portions. The upper, lower tank portions are crosslinked polymer moulding prods. formed by mixing a monomer liq. (A) (soln. (A)) comprising a metathesis polymerisable cyclic olefin contg. a catalyst component of a metathesis polymerisation catalyst system, with a monomer liq. (B) (soln. (B)) comprising a metathesis polymerisable cyclic olefin contg. an activating agent component of the metathesis polymerisation catalyst system, casting the mixed monomer liq. raw material into a mould, and polymerising and crosslinking in the mould. Pref. at least the upper, lower tank portions have rectangular concave, convex ribs (2) in the longitudinal direction. The tanks have 1.5 - 5, on average, of ribs every width of 1mm of the tank in the transverse direction. The respective concave, convex ribs satisfy the following shape characteristics of formulae 10 H 80 (i), 30 W 300 (ii), 2 R &lt;H, 2 R' &lt; H (iii), and 3 t (iv). t (mm) = average thickness of moulding prod. in the section perpendicular to the longitudinal direction of the ribs; H (mm) = height of a convex portion from the surface of the moulding prod.; W (mm) = width of a concave or convex portion; and R, R'(mm) = radius of the curved portion of a concave or convex portion. The concave, convex ribs of the bottom portion of the purifying tank satisfy the above shape characteristics. At least one of the ribs is substantially parallel to the line passing through the flowing-in side and the flowing-out side. In the case that a partitioning plate (8) is so provided in the purifying tank as to reach the bottom of the tank, no concave, convex ribs are formed near the partitioning plate.    USE -   The tank is useful for treatment of domestic effluent and human wastes.    ADVANTAGE -   The tank is superior in lightweight, impact resistance, and durability. It is unnecessary to reinforce with fibres. The recycling is possible. No toxic gas is generated when the tank is east treated.