A wired circuit forming board that can provide improved adhesion between an insulating layer and a conductive pattern and can also prevent delamination in a thin metal layer, a wired circuit board for which the same wired circuit forming board is used, and a thin metal layer forming method for forming the thin metal layer. The thin metal layer 2 is formed on the insulating base layer 1 by sputtering the first metal 35 and the second metal 36 in such a condition that a first metal diffusing region 37 for the first metal 35 to be diffused and a second metal diffusing region 38 for the second metal 36 to be diffused are overlapped with each other. This can allow formation of (i) a first unevenly-distributed metal portion 4 in which the first metal 35 is unevenly distributed and which is adjacent to the insulating base layer 1, (ii) a second unevenly-distributed metal portion 5 in which the second metal 36 is unevenly distributed and which is adjacent to the conductive pattern 6, and (iii) a metal coexisting portion 3 which is interposed between the first unevenly-distributed metal portion 4 and the second unevenly-distributed metal portion 5 and in which the first metal 35 and the second metal 36 are coexistent with each other in such a relation as to be continuously present in the thin metal layer 2 without defining any boundaries therebetween.