We show the existence of involuntary unemployment based on consumers’ utility maximization and firms’ profit maximization behavior under monopolistic competition with increasing, decreasing or constant returns to scale technology using a three-periods overlapping generations (OLG) model with a childhood period as well as younger and older periods, and pay-as-you-go pension for the older generation, and we analyze the effects of fiscal policy financed by tax and budget deficit (or seigniorage) to achieve full-employment under a situation with involuntary unemployment. Under constant prices we show the following results. 1) If the realization of full employment will increase consumers’ disposable income, in order to achieve full-employment from a state with involuntary unemployment, we need budget deficit (Proposition 1). 2) If the full-employment state has been achieved, we need balanced budget to maintain full-employment (Proposition 2). We also consider fiscal policy under inflation or deflation. Additionally, we present a game-theoretic interpretation of involuntary unemployment and full-employment. We also argue that if full employment should be achieved in equilibrium, the instability of equilibrium can be considered to be the cause of involuntary unemployment.