In this study, investigations have been carried out in a detached house with Co-Generation System located in the north of Sendai, Japan. Comparisons have been made to clarify the indoor environment and energy consumption before/after the CGS establishment. Measurement results indicated that indoor air temperature was close before and after. Since CGS was established, the total energy consumption became lower than the same period of the previous year. Gas consumption accounted for about 90% in the total energy consumption. CO_2 emission and energy expense in this detached house reduced to about two third of the total comparing to that in previous year.