A pressure tight large-scaled membrane structure includes a bag body having a plurality of gores attached to each other along an attachment line, and a plurality of first ropes attached over a predetermined range of the bag body in such a manner as to be orthogonal to the attachment line of the gores, whereby a cylindrical shaped portion is formed. When the particles transit the laser beam, light is scattered in all directions and is described by MIE scattering theory for particles about the size of the wavelength of light and larger or Rayleigh Scattering when the particles are smaller than the wavelength of light. Accordingly, an apparatus is provided for finding an optical system setting, a dwell time, and a scan spacing. In addition, the inductor is designed in such a way that the core member envelopes only the coil.