It is an object of this invention to provide an optical waveguide, an optical concentration measuring device, and a method for manufacturing an optical waveguide capable of achieving an improvement of evanescent wave exuding efficiency of propagating light and light extraction efficiency. A core layer provided in an optical waveguide has a first portion having a first film thickness, a second portion having a second film thickness different from the first film thickness, and a third portion connecting the first portion and the second portion. The third portion is formed so that the film thickness is gradually increased from the second portion having the smaller film thickness toward the first portion having the larger film thickness between the first portion and the second portion, and the maximum inclination angle is 10° or more and 45° or less.