A two-melt region bounded by a closed loop in the composition-temperature plane has been found in the liquid S-Te system, which is the first example for a liquid having electrical conductivity. It is located in an extremely narrow range centred at about 39.2 at.% S and 715 degrees C near the threshold of the non-metal-metal transition. It has been shown that structural change in the liquid causes the two-melt phase separation.