Method of manufacturing a display device

In order to suppress defect modes such as shrinkage or unevenness in light-emission in a light emitting element and to shorten a time needed for a pretreatment for forming a layer containing an organic compound (EL layer), according to the present invention, a light emitting element is formed by forming a first electrode that is electrically connected to a source region or a drain region of a thin film transistor, forming an insulating film to cover an edge portion of the first electrode, performing a plasma treatment on the first electrode and the insulating film in an atmosphere containing argon and oxygen, then, forming a layer containing an organic compound (EL layer) over the first electrode and the insulating film, and forming a second electrode over the layer containing an organic compound (EL layer).