Provided is an optical connector adapter of the type in which a shell-side hollow cylindrical portion is integrally formed in a shell, a sleeve holder inserted in the shell has a holder-side hollow cylindrical portion, and a sleeve is held by the shell-side cylindrical portion and the holder-side cylindrical portion. The shell has an inward engaging projection on the inner periphery thereof at its portions facing the holder-side cylindrical portion while the sleeve holder has a holder locking portion. By engaging the holder locking portion with the engaging projection, the sleeve holder is prevented from being removed from the shell. The optical connector adapter is attached to a housing so as to pass through its wall such that the shell-side cylindrical portion is located outside the housing while the holder-side cylindrical portion is located inside the housing.