A two-layer flexible substrate that excels in etching performance and adherence to resist, being free of surface defects. There is provided a two-layer flexible substrate including an insulator film and, superimposed on one or both major surfaces thereof without the use of any adhesive, a copper layer, characterized in that the copper layer has a surface roughness (Ra) of 0.10 to 0.25 [mu]m, and that in the cross section of the copper layer at a position of 1 [mu]m from the insulator film, the average crystal grain diameter is 0.8 [mu]m or less. Preferably, the insulator film is a polyimide film.