To obtain observation images bright both at a near point and a far point by attaching a small-size variable diaphragm device to an objective optical system of an endoscope and making the diaphragm diameter in the objective optical system variable.

The small-size variable diaphragm device consisting of two sheet of polarizing filters 60 and 62 is attached into the objective optical system 38 of the endoscope 10. The polarizing filter 60 is arranged freely rotationally movably around the optical axis P of the objective optical system and a switch 86 is operated to rotationally move the polarizing filter 60 within a range of 90°. When the rotational moving angle of the polarizing filter 60 is set at 0° by operating the switch 86 in making observation at the far point, the diaphragm diameter of a large aperture diameter to realize Fno. necessary for the far point side can be acquired by a diaphragm plate 58. When the rotational moving angle of the polarizing filter 60 is set at 90° by operating the switch 86 in an opposite direction in making observation at the near point, the diaphragm diameter of a small aperture diameter to realize Fno. necessary for the near point side can be acquired by openings 61 and 63 of the polarizing filters 60 and 62.