Sewage plant is urban infra-structure consuming much energy and containing several unused energy sources such as digestion gas and waste heat from sludge-burning plant. Then optimizing the energy budget in sewage plant has been important. In this paper, energy budget in the standard sewage plant was modeled and analyzed. First, simulation results were compared with measured data. Secondly, several parameters related to water and sludge treatment were optimized numerically to minimize total energy consumption in sewage plant. Thirdly, utilization method of digestion gas in the sewage plant was discussed.