The heat transport phenomena near the critical point of fluids were investigated in this study. The very-high thermal compressibility and very-low thermal diffusivity near the critical point of fluids affect thermal energy propagation and lead to the formation of weak acoustic waves as the carrier of thermal energy. The heat transport phenomenon is called the “piston effect”, which is one of the thermodynamic phenomena that occur near the critical point of substances. The piston effect in supercritical nitrogen was investigated using a laser holography interferometer. An experimental apparatus was designed for the visualization study of the piston effect in supercritical nitrogen. Heat was added in step functions from a planar heater in a facedown orientation on the ceiling. The infinite-fringe method with a double-exposure technique was used in this experiment. We successfully observed a heat transport phenomenon piston effect, which is considered to be the 4th mechanism of heat transfer.