A semiconductor device of an embodiment includes a first GaN-based semiconductor layer, a second GaN-based semiconductor layer provided on the first GaN-based semiconductor layer and having a larger bandgap than the first GaN-based semiconductor layer, a source electrode provided on the second GaN-based semiconductor layer, a drain electrode provided on the second GaN-based semiconductor layer, a recess provided between the source electrode and the drain electrode in the second GaN-based semiconductor layer, a gate insulating film provided on a surface of the recess, and a gate electrode provided on the gate insulating film and having an end portion in a gate width direction, located in the recess.