Employment theory does lacks a consensus concerning whether employment variation should be expressed as a change in the hours worked as a representative individual or as a change in the population of employed individuals. By appling the OLG model developed by Lucas [1] and Otaki ([2-4]), the present article describes a serious theoretical conesquence of distinction. The crucial factor that different employment theories are the intertemporal substitution effect and the indivisibility of labor force. Monetary expansion increases the rate of return for money if it is credible in the sense of Otaki [5]. This enhances the hours worked in the representative individual model, and thus, aggregate supply causes demand. Conversely, in the indivisible employees model, such an intertemporal substitution effect does not exist. The monetary expansion directly improves the purchasing power of money and thereby increases the aggregate demand for goods by the older generation. Thus, demand derives supply.