In the decommissioning of nuclear power plants (NPP), we can greatly reduce the amount of low-level waste (LLW) by decontamination process. Generally, chemical decontamination method that use chemical solutions to dissolve the deposited waste is major way of decontamination. But this method takes a lot of time and generates secondary waste. On the other hand, the mechanical decontamination method that projected particles such as metal to decontamination target and remove the deposited waste takes shorter time than chemical decontamination method usually. However, in the case of mechanical decontamination of the small diameter pipe to be used in the primary system of the nuclear reactor is difficult because we must divide it vertically into half, and these small diameter pipes are disposed as LLW. At the decommissioning of advanced thermal reactor Fugen (operation finished in 2003) in Japan, the new decontamination machine which compounded with the blasting machine which projected particles such as metal and the barrel polishing machine which is a machine that polishes like a washing machine using particles such as metal was produced experimentally for the pipes of such a small diameter. By using this machine, the radioactivity of contaminated undivided pipe is decreased, and we found that this machine can use as a decontamination machine. In this study, we operated the blasting and barrel grinder independently in order to elucidate the mechanism of the decontamination system. We placed flat metal simulators in each device and measured mass changes in each to compare which decontamination effect was the primary one. The results suggest that the decontamination effect of the blasting machine is higher than that of the barrel polishing machine. We also used a high-speed camera to observe the decontamination of the blasting system and to investigate the interaction between the pipe sample and the particles. It was suggested that the number of particles entering the inside of the pipe increases as the pipe faces the projection direction.