The disclosed device prevents degradation of distance measurement accuracy due to alignment error in the mounting of a detachable lens barrel on a camera body. The lens barrel houses a focusing objective lens and a first distance measurement position is set along the optical path of the objective lens within the lens barrel. A second distance measuring position is set within the camera body. A light projector is positioned at one of the distance measuring positions and projects a measuring beam, and a photo detector is positioned at the other of the distance measuring positions to receive a reflection beam from the object being measured to produce a signal to be used for focus adjustment.