To prevent non-uniformization of forming conditions when a formed body of a cement-based forming material composed of a cement-containing reverse emulsion composition comprising cement, water and oily material is subjected to age hardening, to sufficiently suppress volatilization of the water and oily material at the time of the age hardening so as to prevent occurrence of dry out, and to make production equipment be commonly used at the time of production of many kinds.

A formed body 1 is arranged inside a sealed vessel 2. A filling material 3 is arranged at the space where the formed body 1 is not arranged in the hermetically sealed vessel 2. The hermetically sealed vessel 2 is heated, and the formed body 1 is subjected to age hardening. The age hardening is performed in a state where the volume of the space 4 around the formed body 1 is reduced, thus the volatilization quantity per area of the formed body 1 can be suppressed. The influence of the atmosphere at the outside of the sealed vessel 2 on the age hardening of the formed body 1 can be prevented. Further, in the case the dimensions or the like of the formed body 1 are changed using the same hermetically sealed vessel 2, when the dimensions or the like of the filling material are changed, the volume of the space 4 around the formed body 1 can be sufficiently reduced.