Provided is an externally heated carbonization furnace that includes a plurality of rotary kilns connected in series, each of which includes an outer cylinder, a kiln inner cylinder that rotate relative to the outer cylinder, and a heater that supplies heating gas to a section between the outer cylinder and the kiln inner cylinder. The externally heated carbonization furnace further includes a drive device that rotates at least one of the kiln inner cylinders and the kiln inner cylinder different from the at least one of the kiln inner cylinders and a control device that controls the drive device according to moisture content of a treated object in the kiln inner cylinder.