Copper alloy plate, plating film-attached copper alloy plate, and methods respectively for manufacturing these products

A copper alloy plate including 0.3 mass % or more and 1.2 mass % or less of Mg, 0.001 mass % or more and 0.2 mass % or less of P, and the balance Cu with inevitable impurities in a thickness center portion in a plate thickness direction; Mg concentration on a plate surface is 30% or less of bulk Mg concentration at the thickness center portion; a surface layer part having a depth from the plate surface to where it is 90% of the bulk Mg concentration is provided; and in the surface layer part, the Mg concentration increases from the plate surface toward the thickness center portion with a concentration gradient 1.8 mass %/μm or more and 50 mass %/μm or less.