A semiconductor device of an embodiment includes: a nitride semiconductor layer including a first GaN region of n-type, a second GaN region of n-type on the first GaN region, a third GaN region of p-type on the first GaN region, a fourth GaN region of p-type sandwiching the second GaN region with the third GaN region, a fifth GaN region of p-type on the third GaN region, a sixth GaN region of p-type sandwiching the second GaN region with the fifth GaN region, a seventh GaN region of n-type on the fifth GaN region, an eighth GaN region of n-type on the sixth GaN region, a trench between the seventh GaN region and the eighth GaN region, the trench having an inclination angle of less than 90 degrees; a gate insulating layer including an aluminum nitride film in the trench; a gate electrode; a first electrode; and a second electrode.