This paper analyzes the stability of a power system in which a synchronous generator and a photovoltaic (PV) generator supply the power to an infinite bus. The infinite bus corresponds to a large system to which the two generators are connected. The problem considered here is to investigate the existence of the equilibrium points of the system and their stability. For the problem, we derive a sufficient condition on the magnitude of the PV current for the existence of the equilibrium points. In addition, we analyze the stability of the equilibrium points, and show that the equilibrium points found under the derived condition are stable. These results clarify the impact of the penetration of the PV generator on the existence of the stable equilibrium points of the system.