We report the relaxation process of photoexcited polydiacetylene in its excitonic states. A coherent polarization involving the B-1(u) excitonic state is generated and probed using the fs-CARS (Coherent anti-Stokes Raman scattering) method by varying the delay time between the pump and Stokes laser pulses. We obtain the relaxation data from an analysis of the wavenumber-resolved anti-Stokes signal. The fast dephasing time in the excitonic states was measured and the geometry relaxation model of the polymer chain is discussed.