An online diagnostic system for a geothermal generation facility is discloses that includes an automatic steam measurement device for measuring a characteristic of steam to be supplied to a steam turbine from a steam-water separator at the geothermal generation facility that outputs analysis data. A monitor-control device controls an operation of the geothermal generation facility while monitoring the geothermal generation facility. A diagnostic device performs at least one of an evaluation of a steam characteristic at the geothermal generation facility, an evaluation of the steam-water separator, and an evaluation of pulsation and confluence of a production well based on the analysis data from the automatic steam measurement device and performance data of the geothermal generation facility from the monitor-control device. An operating status of the geothermal generation facility is diagnosed.