The invention provides a metal substrate and a light source device ensuring that a semiconductor chip working as a light source can be firmly joined by using a metal joining material, such that heat generated in the mounted semiconductor chip can be efficiently dissipated through a metal plate. The metal substrate includes a heat dissipating metal plate made of a metal except for Au, an insulating resin-made white film stacked on a part of the heat dissipating metal plate, and a light source mounting surface-forming layer stacked on another part of the heat dissipating metal plate. The metal substrate is such that the light source mounting surface-forming layer is a metal layer directly contacting the heat dissipating metal plate, and the light source mounting surface is a surface of an Au layer which is the outermost layer of the light source mounting surface-forming layer.