A method for observing a deformation of an elastic material including rubber or elastomer, comprising: a step of preparing the elastic material whose outer peripheral surface is formed into a circular shape, a projection image obtaining step of capturing a projection image of at least a part of the elastic material, from a direction perpendicular to an arbitrary axis of the elastic material, at a plurality of capture positions around the axis, a three-dimensional image constructing step of constructing a three-dimensional image of the elastic material from the projection images captured at the plurality of the capture positions by reversely projecting the projection images, and a step of observing the constructed three-dimensional image, characterized in that the projection image obtaining step comprises: a deforming step of deforming the elastic material in predetermined cycles, the deforming step comprising: a step of rotating the circular outer peripheral surface of the elastic material while pressing onto a contacted surface, a step of rotating a cylindrical drum, wherein the contacted surface is formed in the outer peripheral surface of the drum, and a step of setting a slip angle between the moving direction of the contacted surface of the drum and the moving direction of the outer peripheral surface of the elastic material, a signal output step of outputting an capture signal at a predetermined specific time point during one cycle, and a capturing step for capturing the projection image of the elastic material based on the capture signal, and the deforming step, the signal output step and the capturing step are performed at each of the capture positions. a step of preparing the elastic material whose outer peripheral surface is formed into a circular shape, a projection image obtaining step of capturing a projection image of at least a part of the elastic material, from a direction perpendicular to an arbitrary axis of the elastic material, at a plurality of capture positions around the axis, a three-dimensional image constructing step of constructing a three-dimensional image of the elastic material from the projection images captured at the plurality of the capture positions by reversely projecting the projection images, and a step of observing the constructed three-dimensional image, a deforming step of deforming the elastic material in predetermined cycles, the deforming step comprising: a step of rotating the circular outer peripheral surface of the elastic material while pressing onto a contacted surface, a step of rotating a cylindrical drum, wherein the contacted surface is formed in the outer peripheral surface of the drum, and a step of setting a slip angle between the moving direction of the contacted surface of the drum and the moving direction of the outer peripheral surface of the elastic material, a step of rotating the circular outer peripheral surface of the elastic material while pressing onto a contacted surface, a step of rotating a cylindrical drum, wherein the contacted surface is formed in the outer peripheral surface of the drum, and a step of setting a slip angle between the moving direction of the contacted surface of the drum and the moving direction of the outer peripheral surface of the elastic material, a signal output step of outputting an capture signal at a predetermined specific time point during one cycle, and a capturing step for capturing the projection image of the elastic material based on the capture signal, and the deforming step, the signal output step and the capturing step are performed at each of the capture positions.