No paradox exists in the fact that, in the Special Relativity, for the mass of a material body, considered in motion at constant speed, whose measure is u, the formula: m = m0/√ can be written, while for a photon the same formula holds, when between its source and the observer a state of relative motion at constant speed, whose measure is u, exists and it is observed along the direction perpendicular to the direction of his speed.