A zoom lens system that includes lens groups G1 to G5 that have negative, positive, positive, negative and positive refractive power respectively in the order from a screen side is provided. The fourth lens group G4 includes a cemented lens made up of a negative lens that is concave on the screen side and a convex lens. The zoom lens system satisfies conditions below 1.15

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> Method to improve an extinction ratio of an optical device

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