A performance evaluation method for elastic material including rubber or elastomer, the method comprising: a strain applying step (S2) of applying a strain to a test piece made of an elastic material; an imaging step (S3) of obtaining projected images of the test piece being strained by irradiating X-rays to the test piece ; a detection step (S4) 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; wherein, using the projected images of the test piece , a plurality of tomographic images of the test piece are obtained on an arbitrary plane that intersects perpendicularly to the axial direction of the test piece and thereby a tomographic image of the test piece is composed using the projected images (S41); characterized in that in the detection step (S4) the density of each minute region of the elastic material is measured from the tomographic images , wherein, in the region of the test piece displayed in each tomographic image , a brightness value is acquired for each minute region that constitutes each tomographic image , whereby the brightness value is the lowest in the voids and the higher the brightness value, the higher the density of the elastic material, and whereby thus a proportional relationship is established between the brightness value and the density; a relationship obtaining step (S5) of obtaining a density distribution (R) between the densities and frequency of the low-density regions based on the detected low-density regions ; and a distribution width calculation step (S6) of calculating a distribution width (W) specified by a full width at half maximum FWHM from the density distribution (R) approximated to a normal distribution (D). a strain applying step (S2) of applying a strain to a test piece made of an elastic material; an imaging step (S3) of obtaining projected images of the test piece being strained by irradiating X-rays to the test piece ; a detection step (S4) 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; wherein, using the projected images of the test piece , a plurality of tomographic images of the test piece are obtained on an arbitrary plane that intersects perpendicularly to the axial direction of the test piece and thereby a tomographic image of the test piece is composed using the projected images (S41); characterized in that wherein, using the projected images of the test piece , a plurality of tomographic images of the test piece are obtained on an arbitrary plane that intersects perpendicularly to the axial direction of the test piece and thereby a tomographic image of the test piece is composed using the projected images (S41); characterized in that in the detection step (S4) the density of each minute region of the elastic material is measured from the tomographic images , wherein, in the region of the test piece displayed in each tomographic image , a brightness value is acquired for each minute region that constitutes each tomographic image , whereby the brightness value is the lowest in the voids and the higher the brightness value, the higher the density of the elastic material, and whereby thus a proportional relationship is established between the brightness value and the density; a relationship obtaining step (S5) of obtaining a density distribution (R) between the densities and frequency of the low-density regions based on the detected low-density regions ; and a distribution width calculation step (S6) of calculating a distribution width (W) specified by a full width at half maximum FWHM from the density distribution (R) approximated to a normal distribution (D). <EOS>