To provide an internal residual stress measuring instrument and a method therefor capable of evaluating easily and nondestructively an internal residual stress of a large dimension of actual structure not allowing destructive measurement to grasp a residual service life of equipment, the structure or the like to be operated efficiently.

This internal residual stress measuring instrument and this method is provided with a surface residual stress measuring part 1 for measuring nondestructively a surface residual stress of the measured object, an internal residual stress evaluating part 2 for finding the residual stress in an optional position inside the object, based on a distribution of the surface residual stress of the measured object measured by the surface residual stress measuring part, and a residual stress distribution displaying part 3 for displaying visually the residual stress distribution found by the internal residual stress evaluating part.