A method of manufacturing a display device includes: forming an active layer on a substrate; forming a first insulation layer covering the active layer; forming a gate metal line on the first insulation layer; forming a third insulation layer covering the gate metal line and including a silicon oxide; forming a fourth insulation layer including a silicon nitride on the third insulation layer; forming a fifth insulation layer including a silicon oxide on the fourth insulation layer; arranging a blocking member over a region in which the active layer and the gate metal line overlap; forming a fifth auxiliary insulation layer by doping nitrogen ions in the fifth insulation layer; and exposing a part of an upper surface of the fourth insulation layer by removing a portion of a fifth main insulation layer of the fifth insulation layer which does not overlap the fifth auxiliary insulation layer.