Due to the advances of imaging devices technologies, high quality video has become easy to produce. There are still problems to be solved though. Gaussian noise is always mixed in images by photo-electric conversion. Noise reducer (NR) is a signal processing method to cope with the issue. However, noise level is required for NR to work effectively. Since signal processing methods for video should work in real-times, the noise level also should be detected in real-times as well. In this paper we propose a fast and accurate noise level detection algorithm.