Housedust-mite has been a trigger factor for allergic diseases such as asthma, rhinitis and eczema. Therefore, it is important to remove housedust-mite from the environment of an allergic patient. Eastern red cedar is one kind of cypress tree which grows naturally in Northern America. It is used for the material of pencils, and the essence of soap. Its oil is used for oily substance for microscope lens. Using original system, we examined eastern red cedar and its oil, and found that it is effective for killing and preventing housedust-mites. The result was very effective. In conclusion the eastern red cedar and its oil seems to be useful for controlling mites in the home environment of allergic patients.