The optical properties in poly(isothianaphthene), PITN, have been investigated by photoconductivity measurements and in situ optical absorption spectroscopy measurements during electrochemical n-type doping. Irradiation of the anode induces photoconductive response for a photon energy neighboring the bandgap and upon irradiation of the cathode photoconduction was observed for light of photon energies higher than the band-gap. The origin of this anomalous polarity effect of photoconduction in PITN has been discussed.