It is an object of the present invention to provide a quartz glass jig excellent in the plasma etching resistant characteristics, which does not generate an abnormal etching and particles when used for a plasma generating apparatus. A processing tray receives sheets conveyed from the waiting tray. The blocking surface is moved, when the collar is in the unlocked position, to a position at which the collar permits radial outward deflection of the retention beams. A respective one of the emitters and a respective one of the sensors are disposed at each end of each of the optical fibers.