In forming a mask pattern on a circuit board, a mask pattern of N-layer structure is formed in a region where the mechanical strength of the circuit board needs to be increased. N photosensitive layers are first stacked on a substrate so that they becomes lower in sensitivity from the first photosensitive layer toward the N.sup.th photosensitive layer. In the first photosensitive layer (bottom layer), a first pattern is formed and has the same shape as a predetermined pattern to be formed on the circuit board. In the K.sup.th photosensitive layer (N.gtoreq.K.gtoreq.2), a K.sup.th pattern is formed so that the K.sup.th pattern is smaller than a (K-1).sup.st pattern formed in the (K-1).sup.st photosensitive layer and arranged inside the (K-1).sup.st pattern.