In a glass optical waveguide having a core containing at least one oxide selected from the group of glass-constituting oxides consisting of Bi.sub.2O.sub.3, Sb.sub.2O.sub.3, PbO, SnO.sub.2 and TeO.sub.2, large transmission loss of light which occurs when the cross-sectional shape of the core is rectangle, is reduced, and wherein the glass optical waveguide contains at least total 35% in mass % of at least one type of the above glass-constituting oxides, wherein the cross-sectional shape of the core is trapezoidal, among two parallel sides of the trapezoid, a long side is in a substrate side and among four sides constituting the trapezoid, angles of two oblique sides to the long side are each within a range of from 60 to 80.degree..