This paper shows positive results for introducing energy conservation measures after analyzing facility data acquired by Energy Management System in an existing middle-sized factory. The factory produces around 400,000 gas meters a year. It achieved 37% CO2 reduction from 2011 to 2014. Since 2011, some simple measures had been implemented such as switch-off of unused lights and air-conditioners for unoccupied areas. To achieve higher energy conservation, reduction of air-conditioning load with no drop in productivity and workplace quality was critical. To design new effective energy conservation measures it was necessary to analyze room condition data and energy usage data.