To provide an overspeed protection device for a marine two-shaft gas turbine, effectively preventing the overspeed of an output turbine while maintaining the high responsiveness of the output turbine without requiring excess loading space or loading weight even when air is mixed into a short water suction passage to cause a sudden reduction of load on an output shaft, and its method.

The overspeed protection device for the marine two-shaft gas turbine which has the output turbine 6 as a free turbine to be rotationally driven using exhaust gas from a gas turbine main engine 5 consisting of a compressor, a combustor and the turbine, comprises a speed sensor 12 for detecting a rotating speed N of the output turbine 6 and an overspeed controler 14 for controlling the flow rate of a fuel in the gas turbine main engine 5 depending on the size of the acceleration factor of the detected rotating speed N. The sudden reduction of load is determined from the increasing acceleration of the output turbine to prevent the overspeed.