A refiner (10, 11) for refining fibrous material has a first refining surface (1') and a second refining surface (2') arranged opposite to one another and mobile in relation to one another. The refining surfaces have refining surface portions (15, 27) feeding material to be refined and/or discharging refined material as well as refining surface portions (16) grinding the material to be refined, on the upper surface of which there are blade bars (17) and between them blade grooves (18). In the refining surface (1', 2') of the refiner (10, 11) the cross-sectional area (A) of at least some blade grooves (18) is arranged to decrease from one blade groove (18) to the next from the direction of the feed edge (13) to the direction of the discharge edge (14) of the refining surface (1', 2').