The reason why bio-energy carrier is mainly ATP is clarified by examining the frequencies of Guanine (G) and Cytosine (C) in nucleic acids. Living being uses only the five types of nTPs (ATP, GTP, TTP, CTP, and UTP) for constructing nucleic acids, although only ATP is employed as main carrier of energy. Nucleic acids such as tRNA and rRNA in living being use Guanine with relatively higher densities in comparison with Adenine (A). Thus, mass conservation law teaches us that Adenine is relatively redundant in comparison with Guanine. This leads to the natural consequence that the redundant Adenine becomes energy source as ATP. Next, we will focus on the fact that UTP, GTP, and CTP are also used for generating polysaccharide, protein, and lipid, respectively. We will try to clarify the inevitability of five types of nTPs.