A blade segment for a disc refiner intended for refining fibrous material, which blade segment comprises an inner circumference and an outer circumference as well as a first side edge and a second side edge which combine the inner circumference and the outer circumference, wherein the first side edge and the second side edge of the blade segment are curved so that one side edge is concave and the other side edge is convex, the blade segment has a refining surface comprising first refining surface portions and between them second refining surface portions, the first refining surface portions constituting refining surface portions that are configured to refine the fibrous material to be refined, and the second refining surface portions constituting refining surface portions that run between the first refining surface portions and that are configured to carry the fibrous material to be refined, each of the first refining surface portions comprises first blade bars, the second refining surface portions are constituted by first blade grooves between the first blade bars, and the first blade grooves are configured to carry the fibrous material to be refined from the direction of the inner circumference to the direction of the outer circumference, and the first blade bars and the first blade grooves are curved, the first side edge constitutes the front edge of the blade segment and forms the concave side edge of the blade segment, the second side edge constitutes the back edge of the blade segment and forms the convex side edge of the blade segment, the first blade grooves are curved to the same direction as the front edge, characterized in that each of the first refining surface portions also comprises second blade bars on the upper surface of the first blade bars and constituting between them second blade grooves and in that a width of the first blade grooves on the inner circumference becomes narrower towards the outer circumference. the first side edge and the second side edge of the blade segment are curved so that one side edge is concave and the other side edge is convex, the blade segment has a refining surface comprising first refining surface portions and between them second refining surface portions, the first refining surface portions constituting refining surface portions that are configured to refine the fibrous material to be refined, and the second refining surface portions constituting refining surface portions that run between the first refining surface portions and that are configured to carry the fibrous material to be refined, each of the first refining surface portions comprises first blade bars, the second refining surface portions are constituted by first blade grooves between the first blade bars, and the first blade grooves are configured to carry the fibrous material to be refined from the direction of the inner circumference to the direction of the outer circumference, and the first blade bars and the first blade grooves are curved, the first side edge constitutes the front edge of the blade segment and forms the concave side edge of the blade segment, the second side edge constitutes the back edge of the blade segment and forms the convex side edge of the blade segment, the first blade grooves are curved to the same direction as the front edge, characterized in that each of the first refining surface portions also comprises second blade bars on the upper surface of the first blade bars and constituting between them second blade grooves and in that a width of the first blade grooves on the inner circumference becomes narrower towards the outer circumference.