Abstract: An interpupillary distance adjustment mechanism for an optical device includes a slide rail (1) mounted on a middle support of the optical device, two slide blocks (2) mounted on respective lens barrels of the optical device, and a blocking member (4, 4?). The two slide blocks are slidably movable relative to the slide rail toward or away from each other to change the distance between the two lens barrels of the optical device. The blocking member is manually operable to provide an acting force between the slide block and the slide rail, and thus changing the friction force therebetween during sliding movement of the slide block. In this way, undesired sliding movement of the slide block relative to the slide rail can be prevented.