A method for producing a thin film transistor and a thin film transistor that can suppress deterioration and variation in performance are provided. A method for producing a thin film transistor includes: forming an organic semiconductor layer on a first main surface of a substrate; forming a first conductive layer on the organic semiconductor layer, while forming a second conductive layer on a second main surface of the substrate; forming mask layers collectively on the first conductive layer and the second conductive layer; and bringing the first conductive layer and the second conductive layer collectively into contact with etching liquid so that partial regions of the first conductive layer and the second conductive layer are removed, so as to form a source electrode and a drain electrode on the organic semiconductor layer, while to form a gate electrode on the second main surface of the substrate.