An organic thin film transistor that has good adhesiveness and good contact resistance as well as allows ohmic contact between an organic semiconductor layer and a source electrode and a drain electrode, and its manufacturing method. There is also provided a flat panel display device using the organic thin film transistor. The organic thin film transistor includes a source electrode, a drain electrode, an organic semiconductor layer, a gate insulating layer, and a gate electrode formed on a substrate, and a carrier relay layer including conductive polymer material formed at least between the organic semiconductor layer and the source electrode or the organic semiconductor layer and the drain electrode.