PURPOSE: To supply efficiently illuminating light even in case of a large variable power by providing a negative or positive lens so as to be insertable and detachable between an emitting surface of an optical fiber for supplying the illuminating light, and a view diaphragm, and placing the negative lens and the positive lens at the time of a low magnification and a high magnification, respectively.

CONSTITUTION: In an observing device which is capable of a variable power cation, between a condensing lens 4 for condensing a luminous flux from an optical fiber 1 and an emitting surface 1a of the optical fiber, an aperture diaphragm 2 and a view diaphragm 3 are placed in the vicinity of the emitting surface and on the emitting light side of the aperture diaphragm, respectively. Also, a negative lens 20 is placed between the aperture diaphragm and the view diaphragm at the time of observation of a low magnification, and a positive lens 30 is placed in the vicinity of the aperture diaphragm at the time of observation of high magnification. According to this costitution, it is possible to switch three stages of low magnification illumination for inserting the negative lens, high magnification illumination for inserting the positive lens, and intermediate magnification illumination for inserting no lens.