It is known that the relativistic mass formula is numerically well consistent with the experiment for the charged particles. But, unfortunately, the explanation of the cause of the mass increase is insufficient. We think that the explanation of the cause and effect is very important in physics. It is an axiom that the mass of a body could never be increased unless it does not absorb an outside material, that is, the mass increase is impossible without the absorption of an outside material. In this paper, we add the content of the special theory of relativity by the interpretation of cause of mass increase, paying attention to the idea that the mass of the particle should be taken up as much as mass of the absorbed photon. We very simply derive the mass formula only with the most basic concepts of physics at the last part of this paper.