A process for producing a friction material based on a sheet-like carbon fiber woven fabric for wet-friction elements, such as clutch linings or synchronizing ring linings. In a shooting mode, in a case where an AF frame to be used in the focus control is different from an AF frame to which the set value is set, the controller notifies a user of this fact. The sensor uses two light sources that have two Raman wavelength shifts, wherein the central wavelength of backward anti-Stokes Raman scattering peak of the primary light source coincides with that of the backward Stokes scattering peak center wavelength of the secondary light source, and the time domain reflection signal of the one-way optical fiber Rayleigh scattering is deducted. The selected path expressions are ordered based upon the hierarchy, and a tree structure is created that corresponds to the hierarchy. For \u201creal time\u201d imaging, the frames memory is a fast data storage system such as RAM.