A object is to provide a semiconductor device having normally-off characteristics and capable of easily suppressing field concentration below a side surface of a concave portion. A device includes a nitride-based semiconductor layer having a concave portion formed in a part of one principal surface, a side surface of the concave portion being slanted; a first electrode provided on the principal surface; a second electrode on an opposite side to the first electrode across the concave portion, and provided on the principal surface; an insulating layer formed on both sides of the concave portion in the principal surface; and a control electrode provided on the concave portion and at least a part of the wall surface of the insulating layer on the concave portion-side. A tilt angle of the wall surface of the insulating layer is greater than a tilt angle of the side surface of the concave portion.