The structural variation process of amorphous SiO$_2$ gel upon heat-treatment conditions of 80, 250, 450 and 1000$^{circ}C$ has been studied by using the radial distribution functions (RDF$_{obs}$) estimated from the X-ray diffraction intensities. The expected gel structure was determined by comparing the RDF$_{obs}$ with those for the other six standard samples selected appropriately. The structure of specimens prepared by sol-gel method is well consistent with that of fused SiO$_2$ (${eta}$-cristobalite with cubic symmetry) except a slight difference in O-O band distance.