A pyrolysis apparatus comprises a substantially closed pyrolyzer (4), a supply inlet for supplying pyrolyzable fuel to the pyrolyzer (4), an outlet for taking hot bed material in particle form out of the pyrolyzer, one or more exit outlets (6) for taking condensible gaseous substances separated from the fuel to be pyrolyzed out of the pyrolyzer, a condenser (8) for condensing the condensible gaseous substances into pyrolysis oil, and a line (7) for transferring the condensible gaseous substances from the exit outlet of the pyrolyzer (4) to the condenser (8). The means for maintaining pyrolysisconditions in the pyrolyzer (4) comprise an inlet for taking hot bed material in particle form into the pyrolyzer and means (5) for supplying fluidizing gas to the pyrolyzer. The pyrolyzer (4) is a c hamber bounded directly by a furnace (1) of a bubbling fluidized bed boiler, through which chamber fluidized bed material is arranged to be circulated between the inlet and the outlet. Means (5) for supplying fluidizing gas are divided over an area between the inlet (11) and the outlet (12) in the pyrolyzer (4) so that they create a cross-flow of fluidizing gas in relation to the transfer direction (S) of the bed material and fuel.