A power generation complex plant has a control switch, an overall control unit and a steam bypass facility. The overall control unit determines that a desired steam volume has reached a limit value of the volume of steam to be generated by a steam generating facility. A steam bypass facility control unit adds a bias value B1 to a control command value V4 of the steam bypass facility to generate a new control command value V5 when the desired steam volume is determined to have reached the limit value. The steam bypass facility control unit then controls the volume and pressure of steam passing through the steam bypass facility on the basis of the new control command value V5 so that no switch of control may be made in the control switch.