An optical fiber connection adapter (10) connects connectors (20a, 20b), which each include a ferrule (23a, 23b) that contains a tip of a single-mode optical fiber (22a, 22b), and guides light from a laser light source from the interior to the exterior of a casing. The adapter (10) includes a housing (11), a sleeve (12) configured so that the ferrules (23a, 23b) are inserted therein and so that the single-mode optical fibers (22a, 22b) of the connectors are optically connected to each other, and a dust protection ring (17) disposed at a casing side of the sleeve (12) and configured to regulate rotation of the sleeve (12) relative to the housing (11) and to shield an interval between the interior of the casing and the interior of the housing (11) when the connector (20a) at the casing side is connected.
