Obtaining computer-tomography images of a first section at a distance Z1 from one end surface 112d of a honeycomb structure 100 and a second section at a distance Z2 larger than the distance Z1 from the end surface 112d of the honeycomb structure, determining the existence/nonexistence of a plugging portion 114 in at least one of the plurality of passages based on the computer-tomography image of the first section, determining the existence/nonexistence of the plugging portion in the at least one passage based on the computer-tomography image of the second section, and determining the length of the plugging portion based on the result of determination of the existence/nonexistence of the plugging portion based on the computer-tomography image of the first section and the result of determination of the existence/nonexistence of the plugging portion based on the computer-tomography image of the second section.