PURPOSE: To obtain the title base material having a great difference in the refractive index between a clad and a core and without causing cracking and stripping, by diffusing respective dopant solns. outwardly and inwardly from the inner and outer wall surfaces of a cylindrical silicon porous body, and then sintering the body.

CONSTITUTION: One end face of a cylindrical silica porous body 3 (e.g., a porous body obtained essentially from an alkyl silicate and silica particles by a sol-gel method) is masked with a sealing material 5, and a first dopant soln. 4 for increasing the refractive index of a medium (e.g., tetrabutoxygermanium diluted with ethanol) is charged in the hollow part between both bodies 3. A second dopant soln. 2 for decreasing the refractive index (e.g., tributoxyboron diluted with butanol) is filled on the outside of the porous body 3. Consequently, the first dopant soln. 4 is diffused outwardly from the inner wall surface side of the porous body 3, and the second dopant soln. 2 is diffused inwardly from the outer wall surface side. After diffusion, the porous body is dried, then subjected to dehydration and cell-closing treatment in a gaseous chlorine atmosphere, and then baked to produce a cylindrical transparent glass body.