In recent years, urban areas in Japan suffer from more and more disasters by severe weather such as heavy rain fall or tornadoes. For analyzing and predicting the severe weather, weather radars are expected to observe rapidly developing cumulonimbus by scanning whole three-dimensional space with high spatial resolution; however, conventional weather radars with a parabolic antenna need five to ten minutes for three-dimensional scanning. JRC have developed a prototype of new weather radar equipped with an active phased array antenna using digital beam forming technique. With this phased array weather radar, it takes only 30 seconds to finish a volume scan of the cylindrical space within a radius of 80 km and height of 15 km. We have started experimental operation in Chiba city about 45 km east away from the center of Tokyo, where we had installed the radar in the summer of 2015. In the experiments, we have succeeded in observing severe storm cases, where inside of developing stage of cumulonimbus were clearly revealed.