Abstract: A nitride semiconductor device includes a first semiconductor layer, a second semiconductor layer, a conductive substrate, a first electrode, a second electrode, and a control electrode. The second semiconductor layer is directly bonded to the first semiconductor layer. The conductive substrate is provided on and electrically connected to the first semiconductor layer. The first electrode and the second electrode are provided on and electrically connected to a surface of the second semiconductor layer on a side opposite to the first semiconductor layer. The control electrode is provided on the surface of the second semiconductor layer between the first electrode and the second electrode. The first electrode is electrically connected to a drain electrode of a MOSFET formed of Si. The control electrode is electrically connected to a source electrode of the MOSFET. The conductive substrate is electrically connected to a gate electrode of the MOSFET.