To make diverging light emitted from a light source enter a light guiding member highly efficiently in an illuminating device.

The illuminating device 1 includes: a light source 2 that emits a diverging light; the light guiding member 4 having a core 4a, and a clad 4b disposed on the periphery of the core 4a and lower in refractive index than the core 4a; and a capacitor optical system 3 disposed between the light source 2 and the light guiding member 4 and optically connecting the light source 2 and the light guiding member 4. If the focal distance of the capacitor optical system 3, and a distance from the light emitting part of the light source 2 to the light source side focusing position of the capacitor optical system 3 are taken as f, and DL respectively, the illuminating device satisfies formula (1): 1.5f/DL4.