A gallium nitride semiconductor device is disclosed that can be made by an easy manufacturing method. The device includes a silicon substrate, buffer layers formed on the top surface of the silicon substrate, and gallium nitride grown layers formed thereon. The silicon substrate has trenches 12 formed from the bottom surface, each trench having a depth reaching the gallium nitride grown layer through the silicon substrate and the buffer layers. The inside surface of each of the trenches and the bottom surface of the silicon substrate is covered with a drain electrode as a metal film. The vertical gallium nitride semiconductor device with this structure allows an electric current to flow in the direction of the thickness of the silicon substrate regardless of the resistance values of the gallium nitride grown layers and the buffer layers.