An optical device for displaying a condensing mark includes a light source, and an optical element for condensing light emitted from the light source onto an irradiated surface. The deflection surface of the optical element is divided into a plurality of regions. The light emitted through the respective regions on the deflection surface of the optical element forms partial irradiation images corresponding to the respective regions on the deflection surface of the irradiated surface. The respective partial irradiation images are combined into at least one condensing mark on the irradiated surface.