Wide and small angle X-ray scattering measurements for supercritical fluid Se in the wide density region from liquid to dense vapor have been carried out using synchrotron radiation at SPring-8. The large background for the small angle spectra with the previous high-pressure vessel was much suppressed using a new vessel with a large diamond window. We carefully measured wide angle X-ray scattering near the critical density and obtained the structure factor, S(k), and the pair distribution functions, g(r). By small angle X-ray scattering measurements, we succeeded in observing a medium-range fluctuation in the semiconductor-metal transition in expanded fluid Se