Provided is a miniaturized transistor having high electrical characteristics. The transistor includes a source electrode layer in contact with one side surface of the oxide semiconductor layer in the channel-length direction and a drain electrode layer in contact with the other side surface thereof. The transistor further includes a gate electrode layer in a region overlapping with a channel formation region with a gate insulating layer provided therebetween and a conductive layer having a function as part of the gate electrode layer in a region overlapping with the source electrode layer or the drain electrode layer with the gate insulating layer provided therebetween and in contact with a side surface of the gate electrode layer. With such a structure, an Lov region is formed with a scaled-down channel length maintained.