PURPOSE: To reduce variations of optical characteristics due to temperature variation by using plastic materials which have plural refractive indexes and different temperature variation coefficients.

CONSTITUTION: In a plastic objective lens system consisting of a convex lens, a concave lens, a reflecting surface, etc., when dn1/dt and dn2/dt denote a temperature variation coefficient of a plastic material used for the convex lens and the concave lens, respectively, this lens system is constituted so that a ratio of both satisfies a conditional inequality (10).