A duplex LC communication light detecting adapter for forming two parallel optical transmission lines therein when a duplex LC connector is inserted so as to extract as a leak light a portion of communication light transmitting through the optical transmission lines, including an adapter main body that is opaque to light. The adapter main body includes a first space defined by a first wall and isolated from other spaces to arrange a first optical transmission line of the two optical transmission lines, a first light extraction hole for extracting the leak light from the first space, a second space defined by a second wall and isolated from other spaces to arrange a second optical transmission line of the two optical transmission lines, and a second light extraction hole for extracting the leak light from the second space.