To excellently correct transverse chromatic aberration within the range of specified conditional expressions and to prevent the deterioration of image-formation performance at a peripheral part by making the refractive index and the Abbe number of 5th and 6th lenses constituting a 4th group lens satisfy the specified conditional expressions.

The lens optical system 10 is constituted by arranging a 1st group lens L1 having a positive refractive index, a 2nd group lens L2 having a negative refractive index, a diaphragm 7, a 3rd group lens L3 having the negative refractive index, and the 4th group lens L4 having the positive refractive index in order from an object side to an image side. The 4th group lens L4 is constituted by bonding the 5th lens 5 which is biconcave and the 6th lens 6 which is biconvex and whose strong convex surface faces to the image side. The 5th and the 6th lenses 5 and 6 forming the 4th group lens L4 are constituted to satisfy the conditional expressions 0.005<(n5-n6)<0.035 and -21.0<(ν5-ψ6)<-17.0. In the expressions, n5 means the refractive index of the 5th lens, n6 means the refractive index of the 6th lens, ν5 means the Abbe number of the 5th lens and ν6 means the Abbe number of the 6th lens.