Terahertz (THz) radiation, which exists between microwave and infrared electromagnetic region, has characteristics of penetration without any destruction into materials like radio wave and X ray. This merit enables us to expect that the THz light is useful for non-destructive inspection of industrial products such as plastics and polymers including rubbers. Our research group is investigating possibilities of estimation of various rubber conditions using THz time-domain spectroscopy (THz-TDS). In this article, what kind of information we could obtained inside rubber products by THz radiation is focused and explained based on our previous and new unpublished experimental results.