The ratio between the proton and electron masses is shown to be close to the ratio between the strong and electromagnetic interaction coupling constants at Extremely Low Energy (ELE). Based on the experimental data, this relation has been extended for the weak and gravitational interactions, too. Thus, a mass relation has been found, according to which the rest mass of the Lightest Free Massive Stable Particle (LFMSP), acted upon by a particular interaction, is proportional to the coupling constant of the respective interaction at ELE. On the basis of this mass relation, the electron neutrino and graviton masses have been approximately estimated to 2.1×10^(-4) eV/c^2 and 2.3×10^(-34) eV/c^2, respectively. The last value is of the order of the magnitude of hbar*H/c^2, where H is the Hubble constant and hbar is the reduced Planck constant. It is worth noting that this value has been obtained by fundamental constants only, without consideration of any cosmological models.