The wide research and development on oxide thin-film transistors (TFTs) have led to considerable changes in mainstream technology in various electronic applications. Up to now, much research has been focusing on enhancing the performance of oxide TFTs and simplifying fabricating process. At the stage of research and development in the oxide TFT, unexpectedly high gate current phenomena have been continuously reported by several groups, but the origins have not been yet studied in detail. The unusual gate current interferes with the conductance of the oxide TFT, which makes it difficult to interpret the performance of the TFT. Here we present the origin and control factors of the unconventional gate currents flow in the oxide TFT. The gate current is due to the conduction of electrons through trap sites in insulators, and the current is sophisticatedly controlled by the structural factors of TFT. Furthermore, the gate current flows only in one direction due to the charge state of the oxide semiconductor at the interface with the insulator. We also demonstrate that the vertical current path functions as a diode unit can protect the TFT from unintended gate electrostatic shock.