PROBLEM TO BE SOLVED: To provide a gravity settling tank capable of preventing redeposition of a solute in an extraction solution and adhesion of solid matter on an inner wall, and suppressing a concentration increase of solid matter in a supernatant liquid by retrieving a concentrated solution of the settled solid matter, or the like. SOLUTION: The gravity settling tank 5 comprises: a barrel portion 14 in a tubular form which is configured so that a central axis is in a vertical direction; a bottom portion 15 in an inverted-conical shape which is disposed at a lower end of the barrel portion 14; a stirring blade 16 for stirring the concentrated solution of the settled solid matter, the stirring blade 16 forming a part of a stirrer 11 (a driving unit is not shown in Fig.); a heater (not shown in Fig.) for heating the barrel portion 14 and/or the bottom portion 15; a lid portion 17 for airtightly closing an upper end portion of the barrel portion 14; a slurry supply pipe 18 which is penetrated through the lid portion 17 and extended to a lower potion of the barrel portion 14; and a supernatant liquid discharge pipe 20 which is penetrated through the lid portion 17 and extended to an upper potion of the barrel portion 14, and has an opening portion 19 for inflowing the supernatant liquid separated under a pressurized condition from a horizontal direction at a front end of the pipe 20. COPYRIGHT: (C)2007,JPO&INPIT