A liquid injection and ratio control system is provided for blending an additive into a main flow line at a controlled ratio. The indicator is composed of a light guide with one end surface exposed to a front surface side of the panel and another end surface projecting to a rear surface side of the panel, through the insertion hole, and a light emitting unit provided on the rear surface side of the panel such that a light emitting portion thereof is opposed to the other end surface of the light guide. The beam parameter adjustment mechanism controller may provide the beam parameter adjustment signal based upon a controller function that minimizes the average actual wavelength error or a windowed standard deviation of the actual wavelength error.