PURPOSE: To perform excellent aperture operation regardless of the aperture quantity by composing a diaphragm body of plural crossing light shield plates so taht gaps where illumination light pass are formed, and providing the diaphragm body rotatably.

CONSTITUTION: The diaphragm body 15 of the light source is constituted by stacking four diaphragm plates 41, which are cut along two opposite sides while the two remaining beltlike sides are raises at right angles to form light shield plate 43, where fitting grooves are formed at constant intervals. Those two light shield plates 43 are coupled together by engaging fitting grooves, and two coupled bodies are stacked to constitute the stop body 15. Therefore, the diaphragm body 15 has thickness in an optical axis direction and gaps for illumination light are formed between plate surfaces of the light shield plates 43, which is rotated by a galvanometer to select an aperture angle. Consequently, excellent aperture operation is performed regardless of the aperture quantity without any variation in spectral characteristics.