Discard precedence priority of packets carrying an encoded video stream in a packet network is determined based on priority information included in the encoded video stream. Firstly, a SiO2 pattern layer is formed on a top surface of a substrate. Accordingly, it is possible to reduce a number of seal members for providing sealing between a valve chamber and a decompression chamber and thus to reduce the cost. In addition, at least one of the first lens and the third lens is movable.