Photo-catalyst, which is coated at wall of buildings, is widely used for self-cleaning of it The purpose of this study is to improve combustion efficiency by adding active oxygen generated by photo-catalyst to combustion air. In this paper, the effects of existence of photo-catalyst, moisture within combustion air and irradiation of ultra-violent rays on combustion of carbon monoxide for fuel were investigated. Moreover, the effect of active oxygen generated by reaction of photo-catalyst on combustion of fuel. was considered. As the results, it can be seen that moisture within combustion air strongly affects combustion of fuel, maximum concentration of carbon dioxide CO_2 at combustion air with moisture was greater than one of without moisture.