To provide an apparatus for measuring chromatic dispersion when modulating frequency of a variable wavelength light source is different from modulating frequency of a fixed wavelength light source for reference. This apparatus is provided with a variable wavelength light phase comparator 24 for obtaining a phase difference &phgr;x-&phgr;x′ between a variable wavelength light component and a signal having the first modulating frequency fmx, a fixed wavelength light phase comparator 25 for obtaining a phase difference between a phase difference &phgr;0-&phgr;0′ between a fixed wavelength light component and an electrical signal having the second modulating frequency fm0, a phase difference converter 26 for converting the phase difference &phgr;0-&phgr;0′ calculated by the fixed wavelength light phase comparator 25 into &phgr;ex, which corresponds to the first modulating frequency fmx, and a true phase difference calculator 27 for measuring a true phase difference &phgr; between the phase difference &phgr;x-&phgr;x′ calculated by the variable wavelength light phase comparator 24, and the converted result &phgr;ex of phase difference converter 26, and obtains the true phase difference &phgr; from which an affect of contraction/extension of a DUT 30 is removed when the first modulating frequency fmx and the second modulating frequency fm0 are different, thereby measuring wave dispersion.