To provide a steam turbine power generating unit and its operation method, reducing possibility of malfunction resulting from a slight pressure fluctuation in an MSR, and also protecting pressure of the MSR.

When indicating the full closure of GV13s, and it is detected that pressure detected by a pressure detecting switch 20 is higher than a predetermined pressure value and at least one of GV13s is not in a full closure condition, a pressure rise protecting part 21 instructs MSV12s to fully close so as to perform a turbine trip.