In this study, we improved the Guelph Permeameter (GP) method for the in‐situ measurement of soil hydraulic conductivity. The new device has a simpler structure and a smaller diameter than conventional models, making it easier to insert into narrow wells drilled in the soil layer on mountain slopes. Moreover, numerical simulations replicating GP measurements were conducted using the water retention and permeability data from 250 forest soil samples to propose parameters and correction formulas for practical use. As a result, recommended parameter values for forest soil are presented for the soil constant (α＊) and shape factor (C), which are necessary for calculating the saturated hydraulic conductivity (KS) from the steady‐state infiltration volume (Q). Furthermore, the effects of various measurement conditions on Q such as initial pressure head (ψi), the distance from the bottom of the well to the impermeable layer(S), and water injection time (T), were examined. The effect of ψi on Q was small, except under wet conditions where ψi>－50cm. Since Q is underestimated when S is in the range of 0S/H＜3 (where H is the well water depth), a correction formula was proposed. Additionally, because Q is overestimated if it does not reach a steady state, we propose a method using the infiltration volume at T＝10min (QST10) and a correction formula. Using the proposed device and correction formulas, KS can be measured at any depth on mountain slopes in a relatively short time with sufficient accuracy.