A semiconductor device includes a nitride semiconductor layer, a first electrode and second electrode on the nitride semiconductor layer, a gate electrode, and a gate insulating layer between the nitride semiconductor layer and the gate electrode. The gate insulating layer has a first oxide region containing at least any one element of aluminum and boron, gallium, and silicon. When a distance between the first end portion and the second end portion of the first oxide region is defined as d1, and a position separated by d1/10 from the first end portion toward the second end portion is defined as a first position, an atomic concentration of gallium at the first position is 80% or more and 120% or less of that of the at least any one element.