An optical device guides illuminating light from a light source and includes a light-guiding unit including an incidence face which the illuminating light from the light source is incident upon, an emission face which emits the illuminating light and has a larger area than the incidence face, and a reflection face which guides rays of the illuminating light which are incident from the incidence face to the emission face, and a holder which secures the light-guiding unit in position with the light source. The light-guiding unit and the holder are connected together on the emission face side of the light-guiding unit.