An electric optical adapter unit that is freely detachably attached to a rigid endoscope has an optical system stowed in the center of a housing thereof. An electric circuit such as an automatic iris unit is stowed on the circumference of the optical system. Openings of a frame at both ends of the optical system are sealed hermetically with cover glasses so that the optical system can withstand autoclaving. Electric contact pins fixed in annular plug are hermetically sealed with glass hermetic seals in the back end surface of the frame. In a TV camera unit connected to the back end of the electric optical adapter unit, a CCD and the like are stowed in the center of an imaging optical system housing that is sealed hermetically. A receptacle structured to be watertight is mounted on the circumference of the imaging optical system housing. The receptacle can be freely detachably attached to the electric contact pins of the annular plug, whereby an electric signal can be transmitted.