A filter device (1, 2, 3, 4) provided in a tank for storing liquid, the filter device (1, 2, 3, 4) comprising: a filter case (10, 10A) mounted on the tank and having a bottomed substantially tubular shape with an open upper end; a filter element (20, 20A) provided inside the filter case (10, 10A) and including a first filter medium having a substantially tubular shape; a lid body (30, 30A, 30B) provided at the filter case (10, 10A) and the filter element (20, 20A) to cover the upper end of the filter case (10, 10A), and including an air vent hole configured to cause a space inside the filter element (20, 20A) to communicate with a space outside the filter case (10, 10A); an inflow portion (15c, 14b) configured to cause the liquid to flow into an interior of the filter case (10, 10A) and a space outside the filter element (20, 20A); an outflow portion (40, 40A, 40B, 40C) configured to cause the space inside the filter element (20, 20A) to communicate with the space outside the filter case (10, 10A); and a back pressure valve (70, 70A) provided at the outflow portion (40, 40A, 40B, 40C), wherein the outflow portion (40, 40A, 40B, 40C) includes an upper surface (43a, 45) and a bottom surface (44, 44A, 44B, 44C) provided separated from each other, and a first tubular portion provided protruding in an upward direction from the upper surface (43a, 45) and mounted in a through-hole formed in the filter case (10, 10A), the upper surface (43a, 45) includes a first hole that communicates with a hollow portion of the first tubular portion and through which the liquid flows into the outflow portion (40, 40A, 40B, 40C), an outflow port through which the liquid flows out from the outflow portion (40, 40A, 40B, 40C) is provided in a side surface or the upper surface (43a, 45) of the outflow portion (40, 40A, 40B, 40C), the back pressure valve (70, 70A) includes a valve body (71, 71A) having a substantially plate-like shape, and a first elastic member provided between the bottom surface (44, 44A, 44B, 44C) and the valve body (71, 71A), the valve body (71, 71A) is movable between a closed position in which the valve body (71, 71A) is in contact with the upper surface (43a, 45) and covers the first hole, and an open position in which the valve body (71, 71A) is not in contact with the upper surface (43a, 45), a recessed portion (71b, 71c) is provided in a surface of the valve body (71, 71A), that comes into contact with the upper surface (43a, 45), and in the closed position, a space formed between the recessed portion (71b, 71c) and the upper surface (43a, 45) causes the space inside the filter element (20, 20A) to communicate with the space outside the filter case (10, 10A). a filter case (10, 10A) mounted on the tank and having a bottomed substantially tubular shape with an open upper end; a filter element (20, 20A) provided inside the filter case (10, 10A) and including a first filter medium having a substantially tubular shape; a lid body (30, 30A, 30B) provided at the filter case (10, 10A) and the filter element (20, 20A) to cover the upper end of the filter case (10, 10A), and including an air vent hole configured to cause a space inside the filter element (20, 20A) to communicate with a space outside the filter case (10, 10A); an inflow portion (15c, 14b) configured to cause the liquid to flow into an interior of the filter case (10, 10A) and a space outside the filter element (20, 20A); an outflow portion (40, 40A, 40B, 40C) configured to cause the space inside the filter element (20, 20A) to communicate with the space outside the filter case (10, 10A); and a back pressure valve (70, 70A) provided at the outflow portion (40, 40A, 40B, 40C), wherein the outflow portion (40, 40A, 40B, 40C) includes an upper surface (43a, 45) and a bottom surface (44, 44A, 44B, 44C) provided separated from each other, and a first tubular portion provided protruding in an upward direction from the upper surface (43a, 45) and mounted in a through-hole formed in the filter case (10, 10A), the upper surface (43a, 45) includes a first hole that communicates with a hollow portion of the first tubular portion and through which the liquid flows into the outflow portion (40, 40A, 40B, 40C), an outflow port through which the liquid flows out from the outflow portion (40, 40A, 40B, 40C) is provided in a side surface or the upper surface (43a, 45) of the outflow portion (40, 40A, 40B, 40C), the back pressure valve (70, 70A) includes a valve body (71, 71A) having a substantially plate-like shape, and a first elastic member provided between the bottom surface (44, 44A, 44B, 44C) and the valve body (71, 71A), the valve body (71, 71A) is movable between a closed position in which the valve body (71, 71A) is in contact with the upper surface (43a, 45) and covers the first hole, and an open position in which the valve body (71, 71A) is not in contact with the upper surface (43a, 45), a recessed portion (71b, 71c) is provided in a surface of the valve body (71, 71A), that comes into contact with the upper surface (43a, 45), and in the closed position, a space formed between the recessed portion (71b, 71c) and the upper surface (43a, 45) causes the space inside the filter element (20, 20A) to communicate with the space outside the filter case (10, 10A).