A method of forming a patterned thin film comprises the step of forming a frame having an undercut near the bottom thereof on an electrode film, and the plating step of forming the patterned thin film by plating through the use of the frame. The patterned thin film includes a plurality of linear portions disposed side by side. Each of the linear portions has a portion close to the electrode film. This portion has a width greater than the width of the remaining portion of each of the linear portions.