Recently, various detection systems have been used in various scenes for their respective purposes. They are all different systems. Therefore, the authors proposed a new railway monitoring system by combining the 90GHz band millimeter wave and Radio over Fiber (RoF) technology so as to be able to use the system in various scenes universally. As the millimeter-waves can use a wide bandwidth, high precision detection is possible when they are used for the radar. And, because the radar control units can be grouped in one place by introducing the RoF technology, it is possible to build this system at a low cost. First of all, as the 90GHz band has never been used in the railway environment, the authors carried out the propagation test of the 90GHz band millimeter-wave. As a result, the characteristics of the attenuation of 90GHz band were approximately inversely proportional to the square of the distance and approximately proportional to the amount of rainfall. Then, In order to realize the proposed system, a prototype radar system was made and a field test using the prototype radar system was carried out. It was confirmed that the prototype radar system could detect the rail-way structures, such as the mast, fence, train and building, and walking humans in the track. From these test results, it is expected that the railway monitoring system using the 90GHz band millimeter-wave and RoF technology can be utilized.