It is an object to provide a semiconductor device in which a short-channel effect is suppressed and miniaturization is achieved, and a manufacturing method thereof. A trench is formed in an insulating layer and impurities are added to an oxide semiconductor film in contact with an upper end corner portion of the trench, whereby a source region and a drain region are formed. With the above structure, miniaturization can be achieved. Further, with the trench, a short-channel effect can be suppressed setting the depth of the trench as appropriate even when a distance between a source electrode layer and a drain electrode layer is shortened.