This study classified the life cycle of buildings into construction stage, operation stage, maintenance stage and dissolution/disposal stage and proposed the $CO_2$ assessment method of each stage for the purpose of the life cycle $CO_2$ simple assessment system of apartment house. Specifically, in construction stage, $CO_2$ emission major construction materials that take up over 80% of $CO_2$ emission amount of building construction were selected, and method of evaluating $CO_2$ for 5 standard apartment houses was proposed. In addition, the $CO_2$ emission amounts of building operation stage and maintenance stage were calculated based on "Green Home Certification System" and "Apartment House Building Long Term Repair Plan and Repair Rate" of the Korean Ministry of Land, Transportation and Maritime Affairs, and for dissolution/disposal stage, $CO_2$ assessment method was proposed by arranging the amount of oil use of dissolution equipment and combined equipments into database, and analyzing the process of waste treatment. On the other hand, through the life cycle $CO_2$ assessment simulation on actual building using thus proposed apartment house life cycle $CO_2$ simple assessment program, the potential of applying simple assessment program was evaluated. As a result, the results of material production of construction stage and $CO_2$ calculation by maintenance stage indicated significant results closely approximating 80% of $CO_2$ emission amount by the construction material production of building construction set in this study at first.