PURPOSE: To provide a method for non-destructively and correctly diagnosing remaining life expectancy of a steel product with creep damage caused.

CONSTITUTION: A relation between a creep damage ratio of a steel product and hardness of a carbide part is obtained in advance, a surface of the steel product being used is enlarged to measure the hardness of only the 'carbide part, and from this hardness remaining life expectancy of the steel product is diagnosed based on the above relation. The carbide part is bainite, wherein a small pyramid presser is used to obtain micro Vickers hardness with a test load of 200g or less by means of a microhardness meter for making a recess having a maximum diameter of 50μm or less on a test surface.