The mechanical properties of hypo-eutectic Al-Si alloys are affected by the eutectic structure. It is well known that the eutectic structure forms coarse plate shape when the quantity of P is large. For this reason, a technique for manufacturing hypo-eutectic Al-Si alloy containing a small quantity of P is being developed. Since molten metal treatments such as de-gassing, de-oxidation, modification treatment etc. are carried out in the manufacturing process, we performed analysis of the pre-casting process. It was found that, according to this technique, the quantity of P is as small as 10ppm or less, and it is possible to produce molten metal with relatively small quantity of gas and oxide.