This is a theoretical and experimental study of the water wave reflector which is floating and moored. Solving motions of a moored floating body among waves by the two-dimensional wave theory, we may predict the amplitude of reflected and transmitted wave, so that we may easily find the wave-length at which the incident wave is reflected or transmitted completely. This wave-length depends mainly on the spring constant of its mooring system. Hence, adjusting this restoring force, we may compose a complete reflector or transmitter for an incident wave of arbitrary wave-length. Then we describe conditions of complete reflection and transmission by making use of phases of wave exciting force and mechanical impedance of mooring system in the complex plane. This description enables us to estimate easily the necessary spring constant at an arbitrary wave-length. Experiments on a semi-submerged vertical plate and a circular cylinder show good agreements with theoretical prediction. According to this theory, it becomes possible to design a reflector of long wave over ten times of the breadth of floating body.