Electric energy consumption shares a great portion of commercial building energy. Electric energy saving is essential to reduce total energy consumption in commercial buildings. To draw energy saving methods, it is necessary to monitor real energy consumption patterns and analyze the results. We monitor the lighting and non-lighting energy consumption of eleven zones in a real working office building every fifteen minutes during eleven months. We observe and analyze the monthly and daily energy consumption patterns of all zones and draw several feasible energy saving methods. Moreover, the lighting and occupancy are monitored simultaneously in detail to investigate the unnecessary energy consumption. It shows the possibility of a great amount of energy saving. Because we analyze the energy consumption patterns in all zones, the drawn energy saving methods are applicable to the current building with some added infrastructure and expandable to other similar office buildings. Our result is expected to contribute to reducing the energy consumption in buildings.