Plant has various kinds of functions. One of them is a purifying function of gaseous contaminants including carbon dioxide. It is a noticeable function from the standpoint of protection of global environment. Plant can purify not only carbon dioxide but also nitrogen oxide and sulfur oxide, and the chemicals are used as source of energy after broken-down in the plant. And plant puts out a bioelectric potential which changes depending on the environment and growing condition. In this study, the relationship between the potential and purification capability of a plant is examined. Broccoli is adopted as a subjective plant and the experiment is carried out in five kinds of light frequencies including darkness. As a result, the capability becomes higher and the potential also becomes larger in blue light. A positive correlation is recognized in the relationship between the potential and the capability. A specific gaseous chemical could be purified in particular light-frequency by applying the result of this study.