The invention relates to a device for determining the mechanical leg axis of a femur. Said device is provided with a shaft (3) that comprises, on one of its ends, a securing device (5) that is connected to the shaft and that can be blocked at various angles, in order to obtain a fastening that can be blocked in an angle stable manner and that fastens the shaft (3) to the knee joint of a femur, and comprises a gripping element (10) on its other end. A force-torque measuring device (14) is arranged between the gripping element (10) and the shaft (3), said device enabling the longitudinal and/or transversal force components of force (Fz) exerted via the shaft (3) upon the knee joint to be measured by means of the gripping element (10) and/or torque produced thereby. The invention also relates to a method for determining the mechanical leg axis of a femur by means of the above mentioned device. The shaft (3) is fixed in its position by blocking at various angles, in particular an articulated connection when the measured transversal force components or torque produced thereby is minimized, in particular, is zero.