In a method and an installation for thermally gasifying solid fuel, such as biomass fuel, a layer of the solid fuel forms a fixed bed (13), which is heated so as to form volatile fuel components by pyrolysis. At least part of said volatile components are then caused to move upwardly into a reactor chamber (15), which is defined above the layer of solid fuel, and at least one reactant, preferably comprising oxygen, is fed into said reactor chamber, so that the reactants are contacting and chemically reacting with the volatile components in the reactor chamber. The chemical reaction taking place is preferably an exothermic process so that hot gaseous products are generated and tar components are decomposed. The hot gaseous products from the reactor chamber (15) are discharged through the layer (13) of solid fuel, which are thereby heated so as to maintain the pyrolysis process and so as to gasify char in the fuel bed.