To solve the energy and environmental issues, it is planned that a large amount of photovoltaics (PVs) are introduced in Japan. However, because the characteristic of PV output is different from that of conventional generators, it is possible that the synchronous stability changes due to a large amount of PVs. Additionally, supposing that some generators are stopped when PV output becomes large, decrease of inertia constant and change of power flow are also important factors for synchronous stability. Hence, in this paper, the synchronous stability of power system with a large amount of PVs is studied considering both the voltage characteristic of PV output and the change of system operation such as unit commitment. Specifically, the stability is evaluated from a viewpoint of critical clearing time (CCT) based on both the accelerating and decelerating energy in P-${delta}$ curve. IEEJ-WEST10 model system is utilized as a simulation model.