PURPOSE: To monitor secularly the occurrence of cracks and its progress in material of a steam turbine rotor or the like nondestructively, by fixing a life measuring material consisting of materials, which have a strength equal to or lower than that of material to be monitored, to a material to be monitored and flowing a constant current to this life measuring material to measure the voltage change.

CONSTITUTION: A life monitoring piece 10 is fixed strongly to a center hole part 2 of a rotor 1 of a steam turbine which is a constituted material to be monitored. Changes of stress, distortion, etc. and environments such as a temperature in the life monitoring piece 10 are held in the same state as the center hole part 2. The rotor 1 is rotated, and a constant electric current is flowed to a measuring material 13 of the life monitoring piece 10 from a constant current generator 18 through a converter 17. If a crack 5 in the axial direction occurs in the measuring material 13 as shown in figure, the sectional area of the electric conductive part is reduced, and a potential difference is generated between input line 15 and 15 and is measured through a measuring line 16 by a voltage measuring device 19. This potential difference is increased according as the time elapses from a time t1 when the crack 5 occurs. The progress of the crack 5 appears as the change of a voltage V, and the voltage V is recorded on a recorder 20, and it is detected by a voltage analyzer 21 that the voltage reaches a limit value Vcr at a time t2, and an alarm device 22 is instructed to issue an alarm.