An optical adapter comprises an adapter body and a lens member. The lens member is a cylindrical lens having one or more curved surfaces, and has a function to extend an optical image in a horizontal direction. A taking lens and a viewfinder are arranged vertically, and the optical axes of the taking lens and the viewfinder coincide with each other in the horizontal direction. When the adapter body is attached to the front side of the lens-fitted photo film unit, the front surfaces of the taking lens and the viewfinder are covered with the lens member. The center of the lens member in the horizontal direction coincides with centers of the taking lens and the viewfinder in the horizontal direction.