PURPOSE: To obtain a refractive index type optical system of good performance of which the F value is brighter than 2, by satisfying prescribed conditions in a lens system of 3 group constitution consisting, successively from screen side, of a positive lens, a biconvex positive lens, and a negative lens of which the face on screen side end is a concave face.

CONSTITUTION: This lens consists, successively from screen side to projection tube side, of the 1st group I of positive refracting power, the 2nd group II which has positive refracting power and of which the faces at both ends are convex faces, and the 3rd group III which has negative refracting power and of which the face on screen side end is a concave face, wherein the faces on screen side of at least the 1st group and the 3rd group are aspherical faces and the under-mentioned conditions are satisfied. 1.2f<f1<2.5f, 1.2<f1/f2<2.5, 0.3f<D<0.5f where (f) is the focal length of the entire system, f1, f2 are the focal lengths of the 1st group and the 2nd group, and D is the air spacing between the 2nd group and the 3rd group. Thereby, the refracting type optical system of good performance of which the F value is brighter than 2 is obtained with less number of constituting lenses.