Gate electrodes are formed on a substrate. A gate insulation film is formed so as to cover the gate electrodes. A semiconductor layer is formed in regions on the gate insulation film in a region which overlap with at least the gate electrodes. Plasma treatment is applied to the semiconductor layer using a gas which contains a dopant thus increasing impurity concentration of a surface layer of the semiconductor layer. A conductive film is formed on the surface layer of the semiconductor layer to which the plasma treatment is applied. A source electrode and a drain electrode are formed by etching the conductive film.