Tertiary structures formed by mRNAs impact the efficiency of the translation reaction. Ribosomal frameshift is a well-characterized recoding process that occurs during translation elongation. Pseudoknot and stem-loop structures may stimulate frameshifting by causing a translational halt at a slippery sequence. In this study, we evaluated the efficiency of an unusual -1 frameshift caused by a noncanonical RNA G-quadruplex structure in mammalian cells. The reporter gene construct consisting of a fluorescent protein and Luciferase enabled evaluation of apparent and absolute values of the -1 frameshift efficiency and revealed significant increase of the efficiency by G-quadrupex forming potential sequence. In addition, berberine, a small molecule that binds to and stabilizes G-quadruplex structures, further increased the frameshift efficiency. These results indicate that the stable G-quadruplex structure stimulates the unusual -1 frameshift and has a potential to regulate the frameshift with its ligand.