A plastic objective lens system comprises at least one convex lens constituted of a plastic material exhibiting a relatively small absolute value of the coefficient of change in refractive index with temperature, and at least one concave lens constituted of a plastic material exhibiting an absolute value of the coefficient of change in refractive index with temperature, which is larger than the absolute value of the coefficient of the plastic material constituting the convex lens. A reflecting surface concave to the object and image side may be positioned at the rear of the convex lens and concave lens. The plastic objective lens system decreases fluctuation of the focal length and the conjugate distance due to a change in temperature while aberration is minimized.