Externally heated carbonator mainly indirectly heats sewage sludge, woody biomass and low-grade coal at a high temperature of 300℃ ~ 700℃ under the condition of isolating oxygen to generate calorific value and obtain improved carbide. As a method for producing carbide, there are known high-temperature carbonization by indirectly heating a workpiece at a high temperature of 500-700.degree. C. and semi-carbonization (baking) by indirectly heating the workpiece at about 300.degree. C.. When carbonizing at high temperature, carbides with high gasification rate and self-heating can be produced by ensuring sufficient treatment time at a specified temperature. In the process of semi-carbonization, especially by controlling woody biomass in a very small temperature range, carbides with crushing rate and thermal residual rate can be produced. Furthermore, as an externally heated carbonizer, an externally heated rotary kiln is known, which has an inner kiln barrel rotating around an axis and an outer kiln barrel circulating heating gas around the inner kiln barrel. The externally heated rotary kiln makes the processed object (low-heat substance) move axially in the inner cylinder of the kiln and heat at the same time. Further, there is known a structure in which an externally heated rotary kiln is divided into a front part and a rear part, and a processed product is dried at the front part and carbonized at the rear part.