A permeameter for in-situ measurement of saturated hydraulic conductivity of streambed sediment under a water/sediment interface, comprising: a chamber which has an open lower end and which is inserted into the streambed sediment such that the inside of the chamber is filled with the sediment; first and second hydraulic head-measuring lines (20, 30) connected to a manometer for measuring a hydraulic head difference (Δh); and a water flowing connection line one end of which is communicatingly connected to the chamber, characterized in that, the first and second hydraulic head-measuring lines (20, 30) respectively communicatingly connect to upper and lower portions of an outer circumferential surface of the chamber for measuring a hydraulic head difference (Δh) within the chamber, and the permeameter further comprising: a storage pipe designed to regulate the hydraulic head difference (Δh) within the chamber and to measure a quantity and a flow rate of water flowing through the streambed sediment within the chamber, the storage pipe being installed at a distance from the chamber such that one end of the storage pipe is fixed into the streambed sediment and the other end of the water flowing connection line is communicatingly connected to an inlet of the storage pipe for having the water flowing through the streambed sediment within the chamber and introduced into the inlet via the connection line filling the storage pipe, wherein the storage pipe is inserted into the streambed sediment such that the depth of the inlet is lower than the surface-water surface. a chamber which has an open lower end and which is inserted into the streambed sediment such that the inside of the chamber is filled with the sediment; first and second hydraulic head-measuring lines (20, 30) connected to a manometer for measuring a hydraulic head difference (Δh); and a water flowing connection line one end of which is communicatingly connected to the chamber, the first and second hydraulic head-measuring lines (20, 30) respectively communicatingly connect to upper and lower portions of an outer circumferential surface of the chamber for measuring a hydraulic head difference (Δh) within the chamber, and the permeameter further comprising: a storage pipe designed to regulate the hydraulic head difference (Δh) within the chamber and to measure a quantity and a flow rate of water flowing through the streambed sediment within the chamber, the storage pipe being installed at a distance from the chamber such that one end of the storage pipe is fixed into the streambed sediment and the other end of the water flowing connection line is communicatingly connected to an inlet of the storage pipe for having the water flowing through the streambed sediment within the chamber and introduced into the inlet via the connection line filling the storage pipe, wherein the storage pipe is inserted into the streambed sediment such that the depth of the inlet is lower than the surface-water surface. a storage pipe designed to regulate the hydraulic head difference (Δh) within the chamber and to measure a quantity and a flow rate of water flowing through the streambed sediment within the chamber, the storage pipe being installed at a distance from the chamber such that one end of the storage pipe is fixed into the streambed sediment and the other end of the water flowing connection line is communicatingly connected to an inlet of the storage pipe for having the water flowing through the streambed sediment within the chamber and introduced into the inlet via the connection line filling the storage pipe, wherein the storage pipe is inserted into the streambed sediment such that the depth of the inlet is lower than the surface-water surface.