A dust proofing shutter built-in adapter of an LC type optical connector for interconnecting LC type optical connectors plugs, the adapter comprising: a connector housing; fitting portions (A, B) for LC type optical connector plugs respectively provided in both ends of the connector housing so that optical axes are aligned; a sleeve holder configured to install and retain each of leading end portions of ferrules of LC type optical connector plugs inserted from the fitting portions (A, B) in both ends, the sleeve holder being arranged in an inner portion of the connector housing; a shutter plate which is diagonally arranged and is arranged to close the fitting portion (A, B); a leaf spring which energizes the shutter plate in a closing direction; the shutter plate and the leaf spring being arranged at least within one fitting portion (A, B) of said connector housing; wherein said shutter plate opens against a pressing force of the leaf spring when a leading end portion of a ferrule of an LC type optical connector plug is fitted to the sleeve holder; wherein said shutter plate is provided with a clearance for receiving in a non-contact state the leading end portion of the ferrule of the LC type optical connector plug when the LC type optical connector plug is inserted from a diagonally upward direction side before the fitting portion (A,B); characterized in that a rear wall surface of the clearance protrudes from a rear surface of the shutter plate and the adaptor further comprises an opening portion receiving the rear wall surface of the clearance, wherein the opening portion is formed in a rectangular concave portion, which is formed in an inner surface of a wall of the connector housing while having a magnitude and thickness corresponding to the whole surface of the shutter plate. a connector housing; fitting portions (A, B) for LC type optical connector plugs respectively provided in both ends of the connector housing so that optical axes are aligned; a sleeve holder configured to install and retain each of leading end portions of ferrules of LC type optical connector plugs inserted from the fitting portions (A, B) in both ends, the sleeve holder being arranged in an inner portion of the connector housing; a shutter plate which is diagonally arranged and is arranged to close the fitting portion (A, B); a leaf spring which energizes the shutter plate in a closing direction; the shutter plate and the leaf spring being arranged at least within one fitting portion (A, B) of said connector housing; wherein said shutter plate opens against a pressing force of the leaf spring when a leading end portion of a ferrule of an LC type optical connector plug is fitted to the sleeve holder; wherein said shutter plate is provided with a clearance for receiving in a non-contact state the leading end portion of the ferrule of the LC type optical connector plug when the LC type optical connector plug is inserted from a diagonally upward direction side before the fitting portion (A,B); characterized in that a rear wall surface of the clearance protrudes from a rear surface of the shutter plate and the adaptor further comprises an opening portion receiving the rear wall surface of the clearance, wherein the opening portion is formed in a rectangular concave portion, which is formed in an inner surface of a wall of the connector housing while having a magnitude and thickness corresponding to the whole surface of the shutter plate.