The present invention provides a method for forming a semiconductor region having a desired shape, and also provides a method for manufacturing a semiconductor device with few variations. Moreover, the present invention provides a method for manufacturing a semiconductor device which can reduce the cost with a small number of materials and with high yield. According to the present invention, after a semiconductor film is partially oxidized to form an oxide layer, the semiconductor film is etched using the oxide layer as a mask to form a semiconductor region having a desired shape, and thereafter a semiconductor device using the semiconductor region is manufactured. Thus, a semiconductor region having a desired shape can be formed in a predetermined position without using a known photolithography step using a resist.