A refiner (1, 18, 19) for refining fibrous material has a first refining surface (4, 11) and a second refining surface (4, 11), arranged at least partly substantially opposite to form a refiner chamber (12) to which material to be defibrated is arranged to be fed. The first or second refining surface is arranged to move in relation to the opposite refining surface. The first refining surface has elongated openings (14, 15) through which fibrous material to be refined is arranged to be fed into the refiner chamber, and/or the second refining surface has elongated openings (14, 15) through which fibrous material refined in the refiner chamber is arranged to be discharged. The elongated openings provided through the refining surface are arranged at an angle transverse to the blade bars and blade grooves of the refining surface on the first and/or second refining surface.