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.