Photon relative rest mass is important because it may play a role in particle physics and cosmology. The purpose of the present work is to show conclusively that the photon really has relative rest mass. The argument takes into account the international definition of the reference meter and the empirical value of light speed c0 = 299792458 m/s in local vacuum, and considers the quantity c = 3 x 108 m/s as a specific kinematical parameter having a constant value, with dimension speed. These considerations are combined with the Galilean relativity principle (and its Einsteinian extension), which claimed that absolute rest does not exist; i.e. that ?rest? and uniform motion have only relative character. Then with the help of the latest experiments on light speed reduction to 17 m/s, we find that the photon has relative rest mass. It is worthwhile to notice that within the framework of the formalism developed here, c0 is only a critical speed, not an upper-limit speed like the c in standard special relativity theory.