A semiconductor device and a manufacturing method thereof are provided which enable reduction and enhanced stability of contact resistance between the back surface of a nitride substrate and an electrode formed thereover. A nitride semiconductor device includes an n-type GaN substrate (1) over which a semiconductor element is formed, and an n-electrode (10) as a metal electrode formed over the back surface of the GaN substrate (1). A connection layer (20) is formed between the GaN substrate (1) and the n-electrode (10), and the connection layer (20) is composed of a material that is other than nitride semiconductors and that contains silicon.