It is important to detect the tension of yarn in the spinning process. Contact type tension detectors have been used for this purpose until now. However, the contact type tension detectors lead to the end breakages, changes in the properties of yarn and so on. In this study we tried to solve these problems by developing a non-contact tyep tension detector. We devised the system to convert the frequency of yarn to the tension of yarn and used the photo interrupter to measure the frequency of yarn.