A lens device, in which object light is branched by a half mirror after passing through a photographic lens to allow photographing with two photographing means and in which the half mirror can be retracted from the optical axis to obtain a clear image using one of the photographing means, is provided. The half mirror in the lens device is provided between a front-side relay lens and a rear-side relay lens, and is provided such that the half mirror can be retracted away from an optical axis of the photographic lens and advanced toward from the optical axis. The rear-side relay lens is moved along the optical axis to a position in accordance with the retracting of advancing of the half mirror.