Phase diagram of Au-Si binary alloy system shows a large drop of melting temperature of about 1000 K compared with that of Si at a composition of Au:Si=81:19, where the melting temperature is about 636 K. Mixing of Au and Si below the melting temperature was observed by transmission electron microscopy experiment and found the mixed region show a diffraction pattern of a diffuse ring intensity indicating an amorphous structure of the mixed area. Si L-emission spectra, which reflects the energy state of bonding electrons of Si atom, of molten Au81Si19 alloy was measured for the first time to investigate the energy state of valence electrons of Si. The Si L-emission spectrum showed a characteristic loss of L1 peak, which is related to sp3 directional bonding in crystalline Si. The intensity profile is also different from that of molten Si reported. This suggests a characteristic atomic arrangement exist in the molten state. The intensity profile also indicated a small density of state in the molten state at Fermi energy. The obtained spectrum was compared with the calculated density of state of possible crystal structures reported. The comparison suggested that Si atoms are surrounded by 8 Au atoms in the molten state of Au81Si19 alloy. The formation of this local atomic arrangement can be an origin of a large drop of melting temperature at about Au:Si=81:19.