PURPOSE: To provide a light and durable mortar which is closely stuck to a case and prevents the light mortar from peeling off from the case.

CONSTITUTION: A viscous material or an adhesive (b) having an adhesive force at a normal temperature is applied on a part or the whole surface of the inside face of a rigid synthetic resin case (a) having two side wall faces 1, 2 positioned oppositely and the bottom plate 3 and further, a nearly recess-shaped form. Light mortar (c) is poured in a mold and cured in the case (a) to mold the light mortar together with the case as a unit. A fluid slurry containing light inorganic particles is poured in the light mortar(c). In this way, the synthetic resin case (a) is closely stuck to the light mortar (c) and the light mortar (c) is caught in the adhesive layer to be firmly unified together and prevented from peeling off against the change of temperature and shocks and the reinforced effect of the case is retained and the durability is obtained.