In this study, a planar laser-induced Rayleigh scattering (PLIRS) was applied to unsteady gas jets to understand processes of jet development process and ambient gas entrainment in a relatively high pressure ambient. In the verification of PLIRS, Rayleigh scattered light intensities were in proportion as incident light intensities and molar number densities of scatterer. From the results of this study, it is clarified that dense mixtures occupy in the center part of the jet and large-scale vortices exist around the jet axis as a large-hole nozzle is used, that dense mixtures do not clearly appear as a small-hole nozzle is used, and that in volumetric concentrations in the steady portion of jets distribute according to a law of 3/2.