The remote monitoring diagnostic system and method includes a data storage file used to collect the plant data representing the operation status of a plant and to store the plant data, a monitoring system to monitor the field plant according to the collected plant data, a database storing the past plant data associated with errors having occurred to the plant and actions taken to cope with the errors, a diagnostic system to analyze the plant data sent according to the database, and a reporting system to send a report to the user of the plant regarding the causes for the error and/or actions taken to cope with the error based on the result of the analysis. An error is judged in a combustor, when a difference in temperature of two exhaust gas temperature datum detected from adjacent combustors is larger than a predetermined value.