A semiconductor device in which improvement of a property of holding stored data can be achieved. Further, power consumption of a semiconductor device is reduced. A transistor in which a wide-gap semiconductor material capable of sufficiently reducing the off-state current of a transistor (e.g., an oxide semiconductor material) in a channel formation region is used and which has a trench structure, i.e., a trench for a gate electrode and a trench for element isolation, is provided. The use of a semiconductor material capable of sufficiently reducing the off-state current of a transistor enables data to be held for a long time. Further, since the transistor has the trench for a gate electrode, the occurrence of a short-channel effect can be suppressed by appropriately setting the depth of the trench even when the distance between the source electrode and the drain electrode is decreased.