This disclosure an optical unit having a plurality of structural elements such as lens and a spacer having two or more opposing surfaces of the plurality of structural elements are bonded and secured to each other. A contact portion of the bonding portion is configured to prevent spreading of an adhesive and an adhesive reservoir is provided at the bonding portion. This disclosure further provides an apparatus having an imaging optical system configured to image a side surface in a tubular passage and an illuminating optical system that is arranged centrally within the imaging optical system.