The RGB Bayer filter system consists of mosaic R, G, and B filters on the grid of photo sensors with which typical commercial DSLR (Digital Single Lens Reflex) cameras and CCD/CMOS cameras are taken. Many unique astronomical data taken with RGB Bayer filter systems are available, including transient objects, e.g., supernovae, variable stars, and solar system bodies. The utilization of such data in scientific research strongly requires reliable photometry transformation methods. In this work, we develop a series of formulae to derive magnitudes in the Johnson-Cousins BVR filter system from those in the RGB Bayer filter system.