Abstract: A combined cycle power plant capable of controlling a water level of a boiler drum of the power plant generally comprises a gas turbine system, a system turbine system, and a waste heat recovery system which are thermally linked with a feed water supply line and a steam supply line. The waste heat recovery system includes a boiler which is provided with a drum containing a feed water. The power plant further comprises a drum water level control system for stably maintaining the water level by selectively carrying out respective controlling modes in accordance with operation conditions such as an ordinary operation time, an operation stop time and a loss of fire of the gas turbine system by for example processing control signals in utilization of a change-over switch.