A method of and an apparatus for gasifying carbonaceous material, the apparatus including a gasification reactor, to which oxygen-containing primary gas and secondary gasification gas are introduced, whereby a portion of the material to be gasified is gasified to product gas, residual carbon contained in the fly ash extracted from the product gas is combusted in a cyclone combustor, and the exhaust gas from the cyclone combustor is guided to above the bottom level of the gasification reactor to act as secondary gasification gas.