An evaluation method for stiffness of origami folding is proposed. As a simple idea to evaluate the stiffness, we may consider a sheet of paper as a rectangular board, and evaluate its elastic deformation. However, when we apply this simple idea to folding of a folded sheet of paper, it contradicts the results of a questionnaire survey. We investigate more details of folding motion of origami, and give a hypothesis that the stiffness of folding motion comes from the intersection points of crease lines, where is the toughest points to fold. Our proposed evaluation method is consistent with the questionnaire survey, and hence useful to evaluate the stiffness of folding.