Rheological behavior of silicate melts controls the dynamics of volcanic eruptions. Previous experimental studies have investigated melt viscosity and found non-Newtonian behavior of the melt under a high shear rate. However, the relationship between macroscopic rheology and atomic-scale behavior under shear remains unclear. We developed an experimental system for time-resolved x-ray diffraction (XRD) at high temperature to investigate the atomic-scale structural change in melts under shear. The manufactured deformation apparatus and heating furnace were set on the synchrotron radiation x-ray beamline (BL20XU) of SPring-8; the XRD pattern of the melt at high temperature could be observed using this system because the furnace mainly consists of a boron nitride cylinder with high x-ray transmittance. Here, we report results of fiber elongation experiments for a soda-lime melt. Melt fibers with ∼0.7 mm in diameter and ∼27 mm long were elongated at 100 µm sec-1 at temperatures of 595 °C and 620 °C, and the XRD pattern was obtained every 100 msec. Brittle failure of the melt occurred at 595 °C, whereas the melt viscously elongated at 620 °C. The XRD patterns obtained during elongation did not indicate any clear change immediately before brittle failure. The intensity of the XRD pattern decreased with the elongation at 620 °C, although there was no clear variation in its shape. These results indicate that the atomic-scale structure observed by XRD may not change during the elastic and viscous elongation of the soda-lime melt. This experimental system will be further developed and applied to more polymerized and natural silicate melts.