The computation of environmental load($CO_2$) emissions from a number of inputted materials in a building so far required big consumption of time, cost and manpower, but this research presented a method of computing the $CO_2$ emissions($/m^2$) from all inputted materials in a building only through the selection of major materials and the analysis of major materials as a solution to this. This research proceeded with an analysis of apartment construction projects in a total of 12 apartment complexes among buildings, and selected 15 kinds of major materials and 88 kinds of other materials through the analysis of $CO_2$ emissions($/m^2$) and sensitivity by complexes. The computation result of $CO_2$ emissions($/m^2$) from the selected major materials in 12 complexes showed an error range of maximum 2.3% and average 0.98% from the analysis result with the entire materials, so it was evaluated that the $CO_2$ emissions can be computed through the major materials only.