A vehicle rainfall sensor is made to control the operating speed of wipers depending on rainfall. Therefore this is the apparatus to determine the velocity phase of the wipers roughly based on the amount of rainfall. However, the technology which can judge the size of rainfall amount besides determining speed level of the wipers is developing according to the development of the function of rainfall sensor due to the development of technology. In this study, a rainfall measurement by using light scattering by precipitation particles was used. This measurement is to use light signal reflection from front glass and the bigger particle is the less detection of light by light scattering. The detection area of the rainfall sensor and detection channel were extended sizes to increase the accuracy of the rainfall. Also the W-S-R relational expression was developed by using a relationship between the specific precipitation (R) and the amount of sensor detection (S) when there is speed change of the wipers (W) and an indoor rainfall apparatus was used to convert sensing signal to rainfall. The signal system of vehicle rainfall sensor can be converted to the actual rainfall amount by using this formula and if this is provided to users then the vehicle observation network can produce higher-resolution than actual observation network can be produced.