A conductive film to be a gate
electrode, a first insulating film to be a gate insulating film, a
semiconductor film in which a channel region is formed, and a second insulating film to be a channel protective film are successively formed. With the use of a
resist mask formed by performing
light exposure with the use of a
photomask which is a multi-tone
mask and development, i) in a region without the
resist mask, the second insulating film, the
semiconductor film, the first insulating film, and the conductive film are successively etched, ii) the
resist mask is made to recede by
ashing or the like and only the region of the resist mask with small thickness is removed, so that part of the second insulating film is exposed, and iii) the exposed part of the second insulating film is etched, so that a pair of opening portions is formed.