Gas density variation in an atmospheric-pressure streamer discharge was visualized using the Schlieren method and was simulated by a two-dimensional axisymmetric simulation. The Schlieren images visualized the occurrence of shock waves and decrease in the gas density after the discharge. The axial distribution of the gas density was compared between experimental and simulation models, and we found that the two were in good agreement. Additionally, this paper discusses the effect of the pulse voltage amplitude on the decrease in the gas density. Our simulation results show that the pulse voltage amplitude affects the ratio of energy loss fractions of electrons by electron-impact collision processes.