The imaging optical system (1) has a variable magnification ratio of 1.5 times or more, and in order from the object side to the image side, a first lens group (G 1) having a positive refractive power and a second lens group A second lens group G 2, a third lens group G 3 having a positive refractive power, and a fourth lens group G 4 having a positive refractive power satisfying the conditional expressions (1) and (2) . Ffw is the front focal position at the wide angle end with the contact surface (2a) as the origin, fw is the focal length of the entire system at the wide angle end, fft is the front focal position at the telephoto end with the contact surface (2a) as the origin , Ft is the focal length of the entire system at the telephoto end, h is the maximum image height on the imaging surface (5a), and D is the exit pupil position of the rigid endoscope with the contact surface (2a) as the origin.(1) (ffw-D) / fw2 0.052 / h(2) (fft-D) / ft2 0.052 / h撮像光学系(1)は、1.5倍以上の変倍比を有するとともに、物体側から像側へ向かって順に、正の屈折力の第1レンズ群(G1)と、負の屈折力の第2レンズ群(G2)と、正の屈折力の第3レンズ群(G3)と、正の屈折力の第4レンズ群(G4)とからなり、条件式(1)および(2)満足する。ffwは胴付面(2a)を原点とした広角端での前側焦点位置、fwは広角端における全系の焦点距離、fftは胴付面(2a)を原点とした望遠端での前側焦点位置、ftは望遠端における全系の焦点距離、hは結像面(5a)における最大像高、Dは胴付面(2a)を原点とした硬性内視鏡の射出瞳位置である。(1) |(ffw-D)/fw2| ≦ 0.052/h(2) |(fft-D)/ft2| ≦ 0.052/h
