A control device for a power generation system comprising an energy storage system (ESS) and a power plant is provided. The control device includes: a controller configured to compare a supply frequency of a supply power supplied to a consumer with a reference frequency, compare a charge amount of the ESS with a reference charge amount, and control the ESS and the power plant such that the supply power is controlled according to a result of the frequency comparison and a result of the charge amount comparison, an opening degree control device configured to control an opening degree of a steam valve disposed in the power plant by the control of the controller, and a fuel quantity control device configured to control a quantity of fuel injected into the power plant for power generation by the control of the controller.