This paper presents a planar reinforcement (PR) method for fused deposition modeling. In the proposed PR method, planar reinforcing parts, such as metal sheets or carbon fiber film (CFF), are placed inside the structures laminated by additive manufacturing. The tensile, bending stiffness, and vibration characteristics of the planar reinforced structures were evaluated using mechanical tests (tensile, bending, and impact hammer tests). The mechanical properties of the planar reinforced specimens were compared with those of other specimens built by polylactic acid (PLA) and short carbon fiber reinforcement thermoplastic (SFRTP) filaments. Results show that the planar reinforced structures have greater stiffness than the other specimens. The experimental results are analyzed using the mixture equation of composite materials, and the difference in reinforcement effect caused by metal and CFF sheets is discussed.