In this paper. the powerful jet noise containing discrete frequencies and behavior of pseudo-shock waves in the supersonic jet issuing from a divergent exit had been studied mainly by the flow visualization. The shock waves visualized by a high-speed camera and a Schliren system. The motion of the over-compression regions was studied by the digital image processing. It was clarified that the behavior of over-compression regions was periodical, and those periods were closely related to the discrete frequency of jet noise.