An exemplary nitride-based semiconductor device includes: a nitride-based semiconductor multilayer structure 20 which has a p-type GaN-based semiconductor region whose surface 12 is inclined from the m-plane by an angle of not less than 1° and not more than 5° or the principal surface has a plurality of m-plane steps; and an electrode 30 that is arranged on the p-type GaN-based semiconductor region. The electrode 30 includes a Mg alloy layer 32 which is formed from Mg and metal selected from a group consisting of Pt, Mo, and Pd. The Mg alloy layer 32 is in contact with the surface 12 of the p-type GaN-based semiconductor region of the semiconductor multilayer structure 20.