The biomass, particularly lump wood, is supplied to a shaft reactor at its top and is initially preheated to temperatures of about 150.degree. to 280.degree. C. and dried by a counterflowing hot gas. This is followed by a treatment in an underlying carbonizing zone, the upper portion of which is supplied with hot purging gas at a temperature of 250.degree. to 600.degree. C. The hot purging gas flows downwardly through the carbonizing zone co-currently with the wood. A gas mixture which contains purging gas and gas produced by carbonization is withdrawn from the lower portion of the carbonizing zone and is at least in part combusted outside the shaft reactor to produce a combustion gas, which is used at least in part as hot purging gas.