In our modern biotechnology, our concern is to investigate the mechanic structure and function of human genes. In principle, we can make an ideal type of organic life we want. Accordingly, it may be assumed that the genetic characters of us are no more inherited from our parents, an so not accepted as determined things. If we want, we can remove the bad genetic characters by the negative genetic manipulation, while enrich the good genetic characters by the positive genetic manipulation. Considering utilitarianism, it is very difficult for us to estimate the amount of happiness (or pleasure) and unhappiness (or pain) in relation to the genetic manipulation. Man consists in a very complex organism. With the gene therapy, we can cure a genetic disease, but we may have many unrelated diseases. Although the enrichment of the good genetic character will increase our happiness, such a manipulation may cause more painful unhappiness. Since we don’t know which happiness is more proper for us, we must seriously consider the limitation of the positive side of classical utilitarianism. We, thus, have to take the negative utilitarianism, the least unhappiness of the greatest number, instead of the positive utilitarianism, the greatest happiness of the greatest number. That is, we must focus on “to minimize the pain” rather than to maximize the pleasure. The subject of pain is to be the present as well as future generations. But, we don't know what is true happiness and unhappiness for both generations. According to the negative utilitarianism, the positive genetic manipulation and the germ-line (embryo) gene therapy must be prohibited. To remove our anticipated unhappiness or physical pain, the genetic manipulation must be restricted to only a somatic gene therapy.