Structure of xPbO-(100-X)SiO2 (x = 25-89) glasses has been investigated by means of the X-ray and neutron diffraction and Si-29 MAS NMR measurements. In the radial distribution functions of all the glasses, the Pb-O correlation was observed at 0.23 nm, indicating that the PbO3 trigonal pyramids units do exist in the whole glass forming composition range. Furthermore the existence of the first Pb-Pb correlation at similar to 0.385 nm in the whole composition range suggests that the basic structural unit is considered to be a Pb2O4 unit, which consists of the edge-shared PbO3 trigonal pyramids. These results strongly imply that the Pb2O4 units participate in the glass network constructed by SiO4 tetrahedra even at low PbO content. Differing from other lead-containing glass systems, these structural characteristics of Pb ions in the PbO-SiO2 glass system are responsible for the extremely wide glass-forming region.