Provided is a septic tank for generating methane gas, including: a water-saving type toilet installed in a restroom of a building and discharging excreta of users, wherein high-density excreta is discharged from the water-saving type toilet by using a reduced amount of water to discharge the excreta; an excreta storage tank buried underground and connected to the water-saving type toilet to store the excreta discharged from the water-saving type toilet; a heating unit for heating the excreta storage tank so as to increase a fermentation speed of the excreta in the excreta storage tank; a methane gas supply unit for collecting and discharging methane gas generated as the excreta in the excreta storage tank is fermented; and a gas storage tank connected to the methane gas supply unit to store the methane gas discharged from the methane gas supply unit.
