PURPOSE: To provide a Gauss type lens having a vibration proof function, sufficient brightness, back focusing capability and a superior image forming performance.

CONSTITUTION: At least the lens is provided with a first lens group G1 having a positive refractive power and a second lens group G2 also having a positive refractive power in the order from an object side. A negative lens component L1 having a divergence surface Sd1 whose convex surface faces toward the object side is arranged in the side closest to the object of the group G1. Moreover, a negative lens component L2 having a divergence surface Sd2 whose convex surface faces toward an image side is arranged in the image side of an aperture diaphragm. Furthermore, a displacement means is provided to move the group G2 in the direction approximately orthogonal to an optical axis for vibration proofing. The condition of 0.2<f2/f<4.0 is satisfied when f2 is the focal distance of the group G2 and (f) is the focal distance of the entire lens system.