As speech enhancement for impact noise, it is important to detect the impact noise activity. However, since the impact noise suddenly occurs by hitting an object, a snap and clapping, the impact noise activity cannot be detected easy. Therefore, this paper proposes the impact noise activity detector based on 4th order cumulant. The continuous duration of impact noise is shorter than the one of speech and the power of impact noise sharply varies. Consequently, the distributions of instantaneous power of the impact noise and speech are difference. Therefore, the proposed impact noise activity detection takes advantage of 4th order cumulant which depends on sharpness and skirt of distribution.