The invention relates to a device for needling a web of fiber, said device comprising at least one driven needle beam. A vertical reciprocal movement of the needle beam is carried out by a vertical drive unit and a superposed horizontal reciprocal movement is carried out by a horizontal drive unit or due to a phase adjustment by the vertical drive unit. A weight balancing device is provided for balancing the inertia forces of the crank mechanisms. In order to be able to balance both vertical and horizontal inertia forces in a simple manner, the weight balancing device is formed by at least one balance weight which is associated with the crank mechanism of the vertical drive unit and which is set-off by an angle in the range of <180 DEG from an eccentric element of the crank mechanism.