The infiltration process is one of the most useful fabrication methods for metal matrix composites (MMC). Nevertheless, the wettability between the molten metal and ceramics is usually so poor that pressured infiltration processes have been widely used. However, this process results in high production costs. Therefore, we have been developing a spontaneous infiltration process using an Al-Si-Mg melt and a SiC preform. The SiC preform, fabricated using a water glass binder mixed with Fe2O3 powder, causes a chemical reaction between the melt and the binder. The preform, dipped into the melt, was spontaneously infiltrated, and the infiltration process was confirmed by measurement of the preform weight change during the dipping. Nevertheless, the infiltration mechanism is still not clarified. To clarify the mechanism and further the development, additional experiments, using SiC particles with grain sizes of 30, 68, 220 and 420 μm, were carried out. In the preform, fabricated only with the fine particles, there is no infiltration. Nevertheless, if these fine particles were enclosed within the 420μm layers, spontaneous infiltration occurs. The onset of the infiltration occurred during the breaking of the melt oxide film along with a decrease in the gas pressure by the chemical reaction between the trapped gases in the preform and the Mg and Al in the melt.