To provide an apparatus for measuring the relative index difference between materials of the core section and the clad section in an optical waveguide structure after producng the optical waveguide structure.

The apparatus for measuring the relative index difference in the optical waveguide structure contains: a single wavelength light source which emits light at a single wavelength entering the optical waveguide structure having the core section and the clad section; an imaging means to image the light exiting from the optical waveguide structure; a photographing means to photograph the image produced by the imaging means and to acquire the two-dimensional distribution of the energy density in the image; and a calculating means to calculate the relative index difference between the materials of the core section and the clad section in the optical waveguide structure based on the width of the core section in the optical waveguide structure obtained from the primary differential of the energy density in the image converted into logarithm along a specified direction and based on the gradient in a straight line part of the primary differential.