Method of manufacturing thin film transistor, thin film transistor, and display unit

A method of manufacturing a thin film transistor capable of simplifying the steps is provided. The method of manufacturing a thin film transistor includes the steps of: forming a gate electrode and a gate insulating film sequentially on a substrate; forming an oxide semiconductor film in a shape including a planned channel formation region, a planned source electrode formation region, and a planned drain electrode formation region on the gate insulating film so that the whole oxide semiconductor film has the same carrier density as a carrier density of the planned channel formation region; forming a mask inhibiting heat transmission on the planned channel formation region; and heating the oxide semiconductor film in the air and thereby obtaining a higher carrier density of a region of the oxide semiconductor film not covered with the mask than the carrier density of the planned channel formation region.