Abstract: An illuminating device including an optical light guide in the form of a fibre bundle, and a light source having a condenser lens to incident the incidence end surface of the fibre bundle is disclosed. An optical reflecting body having a peripheral reflecting portion is attached to the incidence end surface of the fibre bundle. Parameters of the lens and the optical reflecting body are determined to decrease light loss and to uniformly distribute light on the incidence end surface. The relation between the parameters is represented as formulae which in one case is applicable when the light intensity distribution of the bright spot image is peaked and in another case is applicable when the distribution is substantially flat.