A performance evaluation method for elastic material including rubber or elastomer, the method comprising: a strain applying step of applying a strain to a test piece (10, 10A, 10B) made of an elastic material; an imaging step of obtaining projected images of the test piece (10, 10A, 10B) being strained by irradiating X-rays to the test piece (10, 10A, 10B); a detection step of detecting low-density regions (15, 15A, 15B) in the test piece (10, 10A, 10B) based on the projected images, wherein each low-density region (15, 15A, 15B) 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; characterized in that in the detection step 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 of obtaining a density distribution between the densities and frequency of the low-density regions (15, 15A, 15B) based on the detected low-density regions (15, 15A, 15B); 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. a strain applying step of applying a strain to a test piece (10, 10A, 10B) made of an elastic material; an imaging step of obtaining projected images of the test piece (10, 10A, 10B) being strained by irradiating X-rays to the test piece (10, 10A, 10B); a detection step of detecting low-density regions (15, 15A, 15B) in the test piece (10, 10A, 10B) based on the projected images, wherein each low-density region (15, 15A, 15B) 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; 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; characterized in that in the detection step 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 of obtaining a density distribution between the densities and frequency of the low-density regions (15, 15A, 15B) based on the detected low-density regions (15, 15A, 15B); 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.