In this paper, the authors propose a method for detecting fire smoke by using the optical flow. This method is not influenced against the image obtainment environment. About 60,000 fires have occurred every year in Japan. To be most important to the fires is an early period fire fighting. At present, the automatic devices of detectiong fires is needed. The alarms which can detect smoke and heat are utilized to house fires. However, these alarms are not useful for the outside of house such as the incendiary or woodland fire. This method is able to detect such a flame that becomes a fire is the early period. First, the region of the flame in the images obtained from the observation camera is detected. Next, the characteristic quantity that expresses the smoke is extracted. This characteristic is not influenced to the motion such as the cloud, leaf and moving objects. In other words, the only smoke can be detected, from the range which looks like the flame in the image.