A method for manufacturing an organic semiconductor device having a gate electrode, a source electrode, a drain electrode, an organic semiconductor layer, a gate insulation layer, and a substrate, including: forming, on the substrate, an underlayer that contains an organic polymer material having a liquid crystal core and is oriented in a specific direction, before forming the organic semiconductor layer; and forming the organic semiconductor layer so as to orient the organic semiconductor layer along the orientation of the underlayer, wherein: the gate insulation layer insulates the source electrode and the drain electrode from the gate electrode; and the substrate supports the gate electrode, the source electrode, the drain electrode, the organic semiconductor layer, and the gate insulation layer.