Plasma is a collection of enormous numbers of charged particles. In other aspects, plasma can also be treated as a special fluid (electromagnetic fluid) interacting with the electromagnetic field by introducing an approximation. When we treat plasmas as an electromagnetic fluid, we can investigate the structure and the motion (equilibrium and stability) of the plasma. In this topic, we learn the principle of the measurement for high temperature plasma and analyze actual measured data in the LHD. When an instability occurs in the electromagnetic fluid, changes in the plasma temperature and the density can be measured as changes in the magnetic field. We analyze the measured magnetic field data using a computer and experience the method of analyzing plasma structure and motion.