To obtain a self-hardening subgrade material early increasing the strength by constituting a burned product of a burned material or a coarsely crushed material thereof including a specified wt.% range of one or more kinds of C11A7CaCl2, C11A7CaF2, and C3A.

A burned product made from one or more kinds of incineration ash of municipal refuse and incineration ash of sewage sludge and composed of 10-40 wt.% of one or more kinds of C11A7CaCl2, C11A7CaF2, and C3A, and one or more kinds of C2S, and C3S is used as a subgrade material. Hereat, if aluminum compound is less than 10-40 wt.%, the incineration ash for aluminum source is in short supply and it is not favorable from the viewpoint of effective use of waste. If the compound exceeds 40 wt.%, an excessive expansion is sometimes brought because of accelerated hydration. It is not favorable too from the viewpoint of the design of a subgrade material. Protection of natural resources such as sand or the like is advanced by selecting clinker for ecocement as a material for a subgrade and the burned product itself brings out early a self-hardening property and increases early the strength of a subgrade.