Disclosed is a method of manufacturing a ZnO-based semiconductor device having at least p-type ZnO-based semiconductor layer, which includes a step of forming a contact metal layer on the p-type ZnO-based semiconductor layer wherein the contact metal layer contains at least one of Ni and Cu; and a step of performing heat treatment of the contact metal layer and the p-type ZnO-based semiconductor layer under an oxygen-free atmosphere to form a mixture layer including elements of the p-type ZnO-based semiconductor layer and the contact metal layer at a boundary region therebetween while maintaining a metal phase layer on a surface of the contact metal layer.