Solid fuel in the form of particles is supplied to a pyrolyzer, and pyrolysis conditions are maintained in the pyrolyzer for separating condensable gaseous substances from the fuel. Heat required by the pyrolysis conditions is supplied at least partly with solid fluidized bed material which passes through the pyrolyzer simultaneously as it is fluidized by fluidizing gas in the pyrolyzer. Condensable gaseous substances separated from the fuel are conveyed from the pyrolyzer to a condenser, in which they are separated as so-called pyrolysis oil in liquid form, and solid fluidized bed material taken from the pyrolyzer and containing combustible pyrolysis residue originating from the fuel is circulated through a gasifier, in which product gas, which is burnt in a boiler or a kiln, is formed from the pyrolysis residue.