Molding sand (S) is put in a mold space defined by a pattern plate (2), having a pattern (2A) on it, and a flask (3). The apparatus includes a thin-plate body (9) and/or a rod (11) to be inserted into the molding sand. It is pre-compacted at a part of it near the inner and outer surfaces of the pattern (2A) and the inner surfaces of the flask (3) by the thin-plate body and rod. The apparatus also has a pressing plate (8) to press all the molding sand after it is pre-compacted.