Cloud plays an important role of the transmission of radiation energy, but it was still remains with uncertainty. It is expected to observe three-dimensional distribution of cloud. Although it is difficult with usual weather radar, Communications Research Laboratory developed an Airborne Cloud Profiling Radar (SPIDER). Its sensitivity is -35 dBZ at 5 km range. In this paper, observation theory and method are discussed. Effectiveness of cloud profiling radar is explained with cloud data, which was obtained with airborne measurement.