An illumination optical system of an image capturing device that is compact and low cost and that achieves a reduction in specular noise is provided. An illumination optical system 20 of an image capturing device applies light to an object 11 to be imaged to capture an image by receiving reflected light from the object 11 to be imaged by means of an image sensor 12. The illumination optical system comprises a plurality of LEDs 14 arranged around the image sensor 12, and a ring-shaped prism plate 22 with a prism surface 24 facing the plurality of LEDs 14 in order to apply light from the plurality of LEDs 14 to the object 11 to be imaged, wherein the prism surface has a row of prisms comprising a number of radially arranged edge lines 23 formed circumferentially.