PURPOSE: To obtain the lens system of less focus movements owing to temperatures by so constituting the lens system as to satisfy the specified conditions by using the lenses composed of the materials of large refractive changes with temperatures.

CONSTITUTION: This comprises using polymethyl methacrylate for the positive the 1st lens L1, polystyrene for the negative the 2nd lens L2 and general optical glass for the positive the 3rd lens L3 subsequently from the object side, making reference temperature +20°C and correcting focus movements with respect to the two points of the temperature -10°C and +50°C. If the combined focal length of the lens system is let (f), the focal length at the temperature T of the i-th lens fi, the incident height of the paraxial ray entering from the object side at the height (f) to the i-th lens component hi and the refractive indices of the i-th lens component are let ni(T), ni(T1), ni(T2) respectively at temperatures T, T1, T2, the temperature dispersion number w1 with respect to the material forming the i-th lens component is defined as formula I and the N pieces of the lens components of the entire system are so constituted as to satisfy formula II.