An object is to apply a transistor using an oxide semiconductor to a logic circuit including an enhancement transistor. The logic circuit includes a depletion transistor 101 and an enhancement transistor 102. The transistors 101 and 102 each include a gate electrode, a gate insulating layer, a first oxide semiconductor layer, a second oxide semiconductor layer, a source electrode, and a drain electrode. The transistor 102 includes a reduction prevention layer provided over a region in the first oxide semiconductor layer between the source electrode and the drain electrode.