A performance evaluation method for elastic material including rubber or elastomer, the method includes a strain applying step of applying a strain to a test piece made of an elastic material, an imaging step of obtaining projected images of the test piece being strained by irradiating X-rays to the test piece, a detection step of detecting low-density regions in the test piece based on the projected images, wherein each low-density region is a region where density of a part of the elastic material becomes lower than that before receiving the strain, a relationship obtaining step of obtaining a density distribution between the densities and frequency of the low-density regions based on the detected low-density regions, and a distribution width calculation step of calculating a distribution width specified by a full width at half maximum FWHM from the density distribution approximated to a normal distribution.