After classifying justice into the second category of “good”—it is welcomed not only for its own sake, but also for its consequences—at the beginning of the Republic bookⅡ (357d4-358a3), Glaucon and Adeimantus then ask Plato to explain this classification more explicitly. The purpose of this paper is to discuss the definition of justice in the Republic and to explain the reason why Plato thinks that justice is itself good and worth having. I would like first to show that Plato does give an account of the value of justice itself by presenting an argument for the definition of justice, and then to demonstrate that Plato’s view of justice properly answers the question proposed by his interlocutors in booksⅡ. In so doing, I hope also to point out the inadequate argument in Prof. M. B. Foster’s paper “A mistake of Plato’s in the Republic”.