Abstract: A digestion system includes: a digestion tank that digests organic matter in excess sludge by anaerobic bacteria; a first piping that supplies the excess sludge to the digestion tank; a second piping that has a larger pipe diameter than a pipe diameter of the first piping; a first reservoir tank that communicates with each of the first piping and the second piping, and that stores excess sludge transported by the second piping; a transporting unit that transports the excess sludge stored in the first reservoir tank to the digestion tank via the first piping, continuously for a predetermined period; and a heater that heats the excess sludge in the first piping by a fluid.