PURPOSE: To provide an unstable thermal stress monitor method enabling unstable thermal stress, applied to the casing of a steam turbine, to be obtained accurately including the unstable state at the start/stop operating time.

CONSTITUTION: Temperature sensors 2 are installed at two different points on a space coordinate assumed in the wall of a turbine casing 1 to measure casing temperature, and the measured temperature is inputted into a computer 5. The surface temperature θi and volume average tempeature θm of the casing 1 are computed by a mathematical difference calculus on the basis of the detection value of each temperature sensor 2, and the computed temperature is introduced into an expression: σ=α.E.β(θi-θm)/1-ν [where α is a stress concentration factor, E is a Young' s modulus, β is the coefficient of thermal expansion and νis a Poisson's ratio] to compute unstable thermal stress σ applied to the casing 1.