PURPOSE: To cut the maintenance cost and operation cost and carry out the temperature control in a flask case in simple and correct manners by obviating the need of a steam generator by heating a flask by generating steam in a flask case.

CONSTITUTION: A dividable flask 1 into which a wax pattern 8 is accommodated and the periphery of which is surrounded by an embedding material 9 is accommodated into a flask case 10, and steam is generated by heating the water or hot water supplied into the flask case 10, and the pattern 8 is softened by heating flasks 1, and the pattern 8 is pushed out from the flask 1 by the pressure of the steam. Then, the pattern space 8a is washed by supplying the hot water into the flask 1, and the mold separating agent is poured into the pattern space 8a, and the pattern space 8a is charged with resin, which is hardened. Since the flask is heated by the steam generated in the flask case, the need of a steam generating device such as a boiler outside is obviated, and the temperature distribution in the flask case can be made nearly uniform, and also temperature control can be carried out in simple and correct manners.