PURPOSE: To enable the photographing in a dark condition, and to facilitate the aberration correction in the case of using a light amplification exchange lens.

CONSTITUTION: The first - sixth lens groups L1-L6 are arranged in order from an object side toward an image-formation side, and a diaphragm is provided between the fourth lens group L4 and the fifth lens group L5. The first lens group L1 is formed out of a lens, of which image side is convex and which has positive refracting power. The second lens group L2 is formed out of a lens, of which both sides are convex and which has positive refracting force. The third lens group L3 is formed out of a lens, of which object side is convex and which has positive refracting power. The fourth lens group L4 is formed out of a first lens 41, of which both sides are convex and which has positive refracting force, and a second lens 42, which is bonded to the first lens 41 and which has negative refracting force. The fifth lens group L5 is formed out of a lens, of which image side is convex and which has positive refracting force. The sixth lens group L6 is formed out of a lens, of which object side is convex and which has positive refracting force.