X-ray diffraction measurements at temperatures up to 1500C and pressures up to 843 bar have been carried out using synchrotron radiation at SPring-8 to investigate structural change in the semiconductor-to-metal transition in expanded fluid-Se. The signal-to-noise ratio in the intensity spectra was better than the previous one using in-house X-ray source. It is found important to estimate the background from a sapphire cell to deduce a structure factor of fluid Se with greater accuracy from the spectra. The preservation of the twofold coordinated chain structure and the contraction of the covalent bond are reconfirmed in the transition. When metallic properties were increased, the first peak in the pair distribution function became asymmetric towards larger distance and the first minimum became larger. These results may imply the modification on the chain structure in the transition.