A color wheel includes lens structures, which are formed on a surface of a substrate opposite to a surface having filter sectors, and which have curved surfaces formed at their abutting ends and positioned along a boundary between adjacent filter sectors. The integrated strength values of each type of product ions are compared among different levels of collision energy, and if there is any significant difference, the energy level corresponding to the largest integrated intensity value is determined as an approximate value. A sealing surface is formed on the inner peripheral surface of the hood part and closely contacts an entire outer periphery of the rubber ring.