Described is a method for manufacturing a semiconductor device. A mask is formed over an insulating film and the mask is reduced in size. An insulating film having a projection is formed using the mask reduced in size, and a transistor whose channel length is reduced is formed using the insulating film having a projection. Further, in manufacturing the transistor, a planarization process is performed on a surface of a gate insulating film which overlaps with a top surface of a fine projection. Thus, the transistor can operate at high speed and the reliability can be improved. In addition, the insulating film is processed into a shape having a projection, whereby a source electrode and a drain electrode can be formed in a self-aligned manner.