This invention provides an organic thin film transistor, which can realize the modification of the surface of a gate insulating layer not only the case where the gate insulating layer is formed of an oxide, but also the case where the gate insulating layer is formed of a material other than the oxide and consequently can significantly improve transistor characteristics, and a method for surface modification of a gate insulating layer in the organic thin film transistor. In an organic thin film transistor comprising a gate insulating layer, an organic semiconductor layer stacked on the gate insulating layer, and an electrode provided on the organic semiconductor layer, a polyparaxylylene layer formed of a continuous polyparaxylylene film is formed on the surface of the gate insulating layer, between the gate insulating layer and the organic semiconductor layer, so as to face and contact with the organic semiconductor layer.