To realize air-tightness protection for a device inside range between an optical constitution member (an eyepiece or an eyepiece receiving pipe) which is movably held and a housing constitution member (an eyepiece lens barrel or a binocular eyepiece lens barrel) which belongs thereto and does not change position at a working position by avoiding defects of well-known solutions and further to realize airtight closing so that an outside of an optical device can be easily cleaned and, at that time, danger of unexpected separation or slippage of the sleeve can be surely eliminated.

An external wall of the eyepiece lens barrel 2 is clamped by an eyepiece side part 5 of the sleeve 1 and an inwall of an opening 7 of the lens barrel housing 3 provided for the eyepiece lens barrel 2 is clamped by a housing side part 6 of the sleeve.