The stainless steel 304 oxidized at $70^{circ}C$ in 2.5M CrO3/5.0M H2SO4 solution and at $200^{circ}C$ , $300^{circ}C$, and $400^{circ}C$ in the air are analyzed with X-ray Photoelectron Spectroscopy (XPS) to obtain depth composition profile of the surface region. It is confirmed that the surface region has a quite different composition from that of the bulk. This is due to a difference in the outward diffusion rates of the oxidized species in the surface region. The order of diffusion rates is Fe > Cr > Ni in the experimental temperature range. In spite of the inferior rate of diffusion, Cr is enriched in the surface when it is oxidized in the CrO3/H2SO4 solution. This is due to preferential dissolution of oxidized Fe.