Exposure to excessive noise is known to cause hair cell damage in the organ of Corti. This has been studied both clinically and in animal experiments and it han been established that the sensory cell and subsequent neural degeneration are the main histologic findings. In this paper, using guined pigs as experimental animals, modes of sensory cell damage by 90 dB machinary noise were observed. As a results, sensory cell degeneration was frequently observed in the outer hair cells and was severe in the basal turn. And in the 3 week experimental groups, sensory cell degeneration was almost not observed or very minimal. However in 6 week experimental groups, cells were more damaged. The electronmicroscopic examination revealed mild proliferation and degeneration of mitochondria, proliferation of the endoplasmic reticulm. The degree of severity on alteration of fine structures were in proportion to the amount of exposed noise.