Abstract: For starting a gasifier for the gasification of carbonaceous fuel with an oxygen-bearing gasification agent in a fluidized bed, the gasifier is filled with an inert gaseous medium and put under an increased pressure. After sealing integrity has been established, a combustible gas mixture is ignited in a combustion chamber connected to the lower part of the gasifier, by means of an ignition device which operates under pressure. The gas mixture is thus burnt, while the increased pressure in the gasifier is maintained, at least until the temperature within the gasifier corresponds to the gasification temperature. The arrangement for carrying the method into effect includes a pressure-resistant combustion chamber for combustion of the combustible gas mixture by means of the ignition device. The combustion chamber is connected at least at times to the lower part of the gasifier, by way of an openable slide member.