To obtain a cross-sectional degree in determining a vibration mode.

This method includes the steps of: carrying out a vibration analysis of a tire (step S1); obtaining a degree component of the tire in a circumferential direction from an analysis result of the vibration result for each direction of vibration (step S2); obtaining the direction of vibration of the tire and the degree of the tire on the circumference from the magnitude of the degree component (step S3); obtaining vibration information on a plurality of observation points, present on a meridian cross-section of the tire, which are antinodes of vibration of the tire (step S4); extracting nodes of vibration at the observation points from changes in the obtained vibration information on the observation points (step S5); and obtaining the cross-sectional degree of vibration of the tire in accordance with the number of nodes thus extracted (step S6).