This paper aims to give Capital in the twenty-first century the theoretical base . This book by Piketty argues on economic inequality in advanced nations. He mainly analyzes it on the point of view of history of economy. So it is insufficient for theoretical research. This paper argues on three points about this book. Firstly it analyzes 'the fundamental force for divergence r > g ' as the principles of accumulation in capitalism. Secondly this paper demonstrates that U-Curve of capital/income ratio is the historical tendency of movement of capital. It examines this on base of Regulation approach. Thirdly it demonstrates that the inequality of labor income emerged as consequences of the global accumulation of capital on the base of the theory of world system. This paper concludes that economic inequality is the inevitable results of progress of modern society.