To shorten the time necessary for hardening and to enhance productivity by filling a hydraulic material containing steel making slag and gypsum as predominant components into a mold at ordinary temperature and curing it under reaction by heating.

The steel making slag is slag in the reducing period of steel making and is a molten mineral produced from calcia, alumina and fluorite so as to remove sulfur in the reducing step of steel making. Anhydrous gypsum, calcined gypsum or gypsum dihydrate may be used as the gypsum. Water is added by such an amount that it is not separated when the above hydraulic material 1 is kneaded. The heating temperature is preferably ≤100°C. The weight ratio between the steel making slag and gypsum is (95:5) to (50:50). The hydraulic material 1 is mixed in a mixer 5. A patterned mold 3 is used as the above mold 2 and a heater 8 is disposed under the drag 6 to cure the material 1 by heating. The cope 7 is dropped and the hydraulic material 1 is pressed. The resultant hardened body 4 is released from the mold, laminated and subjected to secondary curing.