A silica based glass waveguide having almost the same optical characteristics as designed value and an optical module using a silica based glass waveguide having excellent optical characteristics and reduced connecting loss are provided. A silica based glass waveguide is comprising a core waveguide and a clad, a core waveguide is formed rectangular in cross section and of SiO.sub.2--TiO.sub.2 group glass on a synthetic silica glass substrate, and a clad is formed of SiO.sub.2--B.sub.2O.sub.3--P.sub.2O.sub.5 group glass or etc. on a synthetic silica glass substrate and a core waveguide. A synthetic silica glass substrate is formed of pure SiO.sub.2 and having birefringence quantity less than 10 nm/cm.