Rawls' argument for the justification of his two principles of justice is not to prove their truth in the ordinary sense. Instead, he intends to show that we will accept them if we are determined to organize a well-ordered society, where social cooperation is carried on according to the set of rules each member of the society obey voluntarily. In order to demonstrate that, Rawls presents the so-called two kinds of arguments, the contract argument and the coherence argument. I have tried in this article to show that these arguments must not be interpreted separately, but as the two stages of a unified argument. In the contract argument he uses the device of social contract in the original position. This device is an illustrated process we have to follow in order to get the principles acceptable to all the citizens of modern democratic society. In the coherence argument, the principles chosen are checked by referring to our considered judgments, because we are supposed to want to have a set of rules compatible with them. In the case of conflict between the principles and judgments, we need to reconsider the process of contract and our judgments. Rawls hopes that, by going back and forth between the two stages, we will reach the reflective equilibrium and accept his principles of justice.