PURPOSE: To detect whether erosion and damage exist or not in the first stage of a nozzle, without disassembling a turbine, by monitoring thermal efficiency of the nozzle of the first stage of the turbine.

CONSTITUTION: A steam temperature and pressure Ti, Pi of an inlet of a main steam stop valve, and a steam temperature and pressure PB, PS of an output side of a nozzle of the first stage of a turbine are stored in a computer, and in accordance with a steam table, each enthalpies Hi, Hs and enthalpy Ho after adiabatic change are decided. As a result, heat drops UE, AE by the enthalpies Hi and Hs, and Hi and Ho are decided. Subsequently, thermal efficiency η is calculated, thermal efficiency η corresponding to a steam regulating valve cam angle is decided by an output from a steam regulating valve cam angle detector 27, is compared with corresponding thermal efficiency η1 of the normal time by a discriminator, and in case when its result is an allowable valve or above, generation of damage, etc. is decided. In this way, whether erosion and damage exist or not in the first stage of the nozzle of the turbine can be detected without disassembling the turbine.