Binoculars having a fixed lens mount, a movable lens mount which is movable straight in a direction as approaching or moving away from the fixed lens mount to adjust their distance to viewer's eyes, a cover member for concealing a space formed between the fixed lens mount and the movable lens mount in accordance with a moving state of the movable lens mount. To the fixed lens mount is attached a first guide engaging with the movable lens mount to slidably guide the movable lens mount as well as a second guide engaging with the cover member so that the cover member is slidable along the second guide together with the movement of the movable lens mount. The binocular further includes a focus adjusting system between both lens mounts to move an objective lens incorporated in each lens mount in a direction of an optical axis, and an erect prism unit including a roof prism in each lens mount. The each roof prism is arranged inner side of respective lens mount so that an inclined cutting portion thereof faces each other. A guide shaft for guiding the object lens in a direction of the optical axis is provided within a space formed in the vicinity of the inclined cutting portion of the roof prism.