PURPOSE: To provide a bright ralay optical system comparatively small in size and high in image forming performance.

CONSTITUTION: This system is provided with a first lens group G1 having comparatively weak negative refractive power, a second lens group G2 having a positive refractive power, a third lens group G3 having a positive refractive power and a fourth lens group G4 having comparatively weak negative refractive power. An aperutre diaphragm is provided between the second lens group G2 and the third lens group G3, the first lens group G1 and the second lens group G2; the third lens group G3 and the fourth lens group G4 are symmetrically arranged to the aperutre diaphragm and the conditions: -0.3<f/f1<-0.1 and 0.5<f/f2<1.1 are satisfied, where, f, fl, f2 are focal distances of a whole lens syste, the first lens group G1 and the second lens group G2, respectively.