To provide an inner focus type telephoto lens that is as small as about 1.8 in F number and is small and lightweight.

An inner focus type telephoto lens comprises, in order from an object side to an image surface side, a first lens group G1 of positive refractive power, a second lens group G2 of negative refractive power, and a third lens group G3 of positive refractive power. The first lens group G1 comprises, in order from the object side to the image surface side; a 1a-th lens group G1a having at least three positive lenses and at least one negative lens, and having positive refractive power as a whole; an aperture stop S; and a 1b-th lens group G1b having at least one positive lens. The second lens group G2 moves towards the image surface side when focusing takes place from an infinity object to a proximity object.