A substrate for a printed circuit board according to an embodiment of the present invention includes a resin film and a metal layer stacked on at least one of surfaces of the resin film. An average diffusion depth of a main metal of the metal layer in the resin film is 100 nm or less after a weather resistance test in which the substrate is held at 150° C. for seven days. The average diffusion depth is preferably 80 nm or less before the weather resistance test.