A system for treating excrement of livestock includes: reduced-pressure fermentation drying equipment configured to store excrement of livestock in an airtight container, heat and stir the excrement of livestock under reduced pressure so that a temperature of the excrement of livestock is within a predetermined temperature range, decompose organic components of organic matter using microorganisms, and obtain volume-reduced dried product; and heat source equipment that is provided on a downstream side of the reduced-pressure fermentation drying equipment and generates a heat source by combusting the obtained volume-reduced dried product.