To obtain a waveguide of a low loss with a large difference of specified refractive index by heating the waveguide in a process of heat treatment to heat to a specified temp. range in the production process of the waveguide so that a part of fluorine is diffused and transferred from a core embedding layer in contact with a core.

This waveguide has such a structure that intermediate layers 3, 5 which is embedded so as to cover the periphery of a core 4 having an almost square cross section, and SiO2-based clad layers 2, 6 which is embedded so as to cover the periphery of the intermediate layers 3, 5 (core embedding layers) are formed on a substrate 1. The core 4 consists of a SiO2 glass or SiO2NyHz glass having high refractive index with addition of a component to control the refractive index (dopants to increase the refractive index such as GeO2 and TiO2). The intermediate layers consist of a fluorine-doped SiO2 glass having a low refractive index. In the production process of the waveguide above described, the waveguide is heated once or more times in a heat treatment process to the temp, range of 800 to 1200°C so that a part of fluorine is diffused and transferred from the core embedding layers 3, 5 in contact with the core.