A safety operation method includes detecting a steam pressure inside a high-pressure casing of the high-pressure part and a steam pressure inside a low-pressure casing of the low-pressure part; obtaining a low-pressure casing limit pressure as a reference corresponding to a pressure of the high-pressure casing in each detection, on a basis of a pressure correlation line expressing a prescribed special relation between preset high-pressure casing pressure and low-pressure casing pressure of the extraction steam turbine; comparing the low-pressure casing limit pressure with the detected pressure of the low-pressure casing; and forcibly throttling an opening of the main steam control valve to reduce the flow rate of steam flowing into the high-pressure part, in a state in which the extraction control valves continue controlling an operation of the extraction steam pressure, when the detected pressure of the low-pressure casing is judged to be higher than the low-pressure casing limit pressure.