In this paper we reconsider and extend the economic theory of deep depression, initiated by Keynes (1936), taking explicit account of the concept of public goods largely blurred in his general theory and after. To contrive the model of secular stagnation where the interest rate falls to zero and monetary policy is restricted by the zero-bound, we concentrate on "real economy" model featuring the theory of public finance. In so doing, we synthesize the traditional theory of public finance and the Keynesian theory of fiscal policy; and cast new light to their welfare significance. In stead of the Samuelsonian neo-classical synthesis, we propose the neo-Keynesian (or the mixed economy) synthesis that requires optimization of public good provision as well as full employment of labor.