A blade segment of a disc refiner for refining fibrous material has an inner circumference and an outer circumference as well as a first side edge and a second side edge which connect the inner circumference and the outer circumference. The side edges are curved, with one convex and the other concave. The segment has a refining surface with blade bars and grooves which define a pumping direction. The side edges of the blade segment curve in the vicinity of the inner circumference in the pumping direction, and in the vicinity of the outer circumference in a non-pumping direction.