In a semiconductor device including a semiconductor element and a wiring substrate on which the semiconductor element is mounted. The wiring substrate includes an insulating substrate and conductive wiring formed in the insulating substrate and electrically connected to the semiconductor element. The conductive wiring includes an underlying layer formed on the insulating substrate, a main conductive layer formed on the underlying layer, and an electrode layer covering side surfaces of the underlying layer and side surfaces and an upper surface of the main conductive layer. The underlying layer includes an adhesion layer being formed in contact with the insulating substrate and containing an alloy of Ti.