The present invention relates to an optical mask at low cost and so on having a structure capable of reducing further a peak of beam intensity caused by diffraction. The optical mask is constructed by overlapping a first mask and a second mask. The first mask and second mask each are a serrated aperture mask. The schematic shapes of the respective apertures of the first mask and second mask are almost rectangular and identical to each other. The fringe defining the aperture of the first mask is machined in a serrated shape, and the fringe defining the aperture of the second mask also is machined in a serrated shape. In the optical mask, the first mask and second mask are overlapped so that the serrations of the fringes defining the respective apertures of the first mask and second mask are located alternately.