1. The writer of this report acknowledge the unique contribution of Lewin and Kounin's Rigidity Theory to theorical developments of “Developmental Psychology”. Having found that the behavior traits as referred to by the two psychologists as the phenomenal rigidity, are common to some behavior traits of the old, the writer tried to explain the old-the last stage of development- on the basis of this rigidity concept while endeavoring to verify the concept itself 2. The first puzzle experiment is intended to examine the degree of influence which the solution acquired in the first series of this experiment may have upon the correspondent solution in the third series as the former is considered likely to hamper the latter with declining effects. 3. The second and third card classification experiments are designed to classify the ability of the subjects to change the classification standard either by themselves or upon instruction from the experimenter. 4. This report covered in its serial experiments between 11 and 13 old age (age 63-82 and residents of an old people's home) plus 16 young subjects (age 14). 5. Following are princ pal findings of the above experiments: a. It was found through the first experiment that the old age took less advantage of solution-hints in solving the given puzzles than the young subjects. Attention in called to the fact that the old age who used somehow or the other the solution-hints in the first series of the test did hardly pay attention to them in the initial part of the third series. The writer's clue to this problem is as follows: Assuming that the mental material of the subjects is rigi, it follows that they are liable to act in their immediate-mindedness without being influenced by their past experiences. Their immediate-mindedness-a mental condition in which one is inclined to discard all preceding developments in favor of what is going on at sight-had led them to overlook the significance of the solution hints. b. In the second and third experiments, it was found that the old hardly changed the classification standard. Such a tendency to persevere in one method and consequently refrain from trying any other solution explains itself for the rigidity concept. In other words, the presence of rigid mental material makes it difficult for the old to re-arrange the once established whole relations of each region in accordance with situational changes. 6. It was observed that the old showed the behavior traits which answer to some extent the description given by Kounin in his study of the feebleminded people. 7. All these findings have led the writer to belive that the rigidity concept is useful for the explanation of some behavior traits of the old. But this does not mean that the writer considers that concept to be the only and sufficient interpretation. This being so, the writer suggests that further efforts should be made to examine the whole concept carefully against eventual accumulation of factual evidences.