An angle-tunable wedge degrader system has been developed to be used as a monoenergetic degrader for a low-energy radioactive isotope (RI) beamline working with the slowing-down method. The system consists of a pair of aluminum degraders with quadratic cross sections, whose overlap works effectively as a wedge-type degrader; the wedge angle can be tuned by changing the relative position of the two aluminum degraders. This paper presents the operation principle, estimation of performance and application range by simulation, and the experimental validation.