JPS632450B

PURPOSE:To prevent contamination, by providing a means, which cleans external air and supplies the air by every specified amount, supplying the external air to an optical system, and cleaning the system. CONSTITUTION:When intake air is flowed and vortexes 25 are yielded at the side of an opening 14a of a vortex yielding body 10, the pressure at the part of the side of the opening 14a is decreased to the lower value than the pressure on the side of an opening 14b. Thus the difference in pressure between both openings is yielded. Based on said pressure difference, a vibrator 17a performs one outgoing and returning twisting vibration with regard to the yield of a pair of vortexes. The variation in the amount of reflected light accompanied by the variation in said vibration is detected by a light receiving element 18 through an optical fiber 13a, and the vortex frequency is measured. The amount of the intake air is obtained from the frequency. The pressure in a chamber 16 is always negative during the period an automobile is driven and the intake air is flowed. The clean air is always supplied to a light transmitting part from a purging device 22. Therefore the optical system is cleaned and its contamination can be prevented.