This study is to examine the emission rate of $CO_2$gas as the environmental load in office buildings. After the investigation of monthly consumption of each energy source(electricity and natural gas), it is analyzed that the $CO_2$emission rate of 34 office buildings surveyed is 22.4kg-$c/m^2$.year, which consists of 17.5kg-$c/m^2$.year by consuming electricity, and 4.9kg-$c/m^2$.year by consuming natural gas. And the $CO_2$emission rate of each load in those buildings consists of 68% emitted by general electricity, 16% by cooling load and 16% by heating load. It is also proposed that the $CO_2$emission rate of cooling and heating load is profoundly pertinent to the variation of outdoor temperature.