Today, the methane fermentation is noticed as a very useful conversion technology of the biomass to recover energy. The product gas (the biogas) mainly consists of methane and carbon dioxide. In the methane fermentation, a large amount of sludge is yielded and the valuable utilization of the sludge is desired. And then we prepared the sludge char at several carbonization temperatures (673-1273 K) and examined the recovery of methane from biogas by using the sludge char. And also we tried to use the sludge char as a humidity control device. We investigated the influence of the carbonization temperature on the pore structure of the sludge char and on the adsorption property of the water vapor on the char. The pore size distribution of the char prepared at 673-773 K was very sharp and the range was between 0.40 and 0.43 nm. The adsorption rate of carbon dioxide was 20 times faster than that of methane on the char prepared at 773 K and then it was found that this char was suitable for the recovery of methane from biogas. In the adsorption isotherm of the water vapor on the sludge char, the rapid increase of the amount adsorbed was found. Therefore, it was found that the sludge char is suitable for the humidity control device.