What is the reason Homans' advocacy of behavior-reductionism, psychological-reductionism, is not popularly accepted among the sociologists? Considering the fact that his assertion is one of the pioneer attempts to advance the level of sociology from descriptive-taxonomic one to propositional-theoretical one, such a state is, I think, unfortunate for sociology. We should say that, in part, Homans himself is responsible for the state. Because, in advocacy, he is limited to draw the reductionism as a necessary logical conclusion, without making effort to eliminate any misunderstanding and detestation of the reductionism. Accordingly, our first task is to eliminate misunderstandings and detestation of the reductionism, and our next task is to advocate adopting psychological reductionism by emphasizing the significance of exercising it. To perfom our task, main accounts would draw upon the following points. 1. To grasp the essential ideas which constitute the basis of his assertion of psychological reductionism. a) Reduction, in his use of the word, is considered as a kind of deduction. What distinguishes his deduction from others is the point that the relation between explanante and explicandum is not from perspective of generarity-specificity but from perspective of elementality-collectivity, more universalityless universality. b) There is two kinds relations of explicandum and explanante. Sociological propositions have a double role : they are in one case explanantes, and in another case explicanda. Sociological propositions, structural proposition, become explanantes when they explain the occurence of social phenomena, and become also explicanda when they are explained by psychological propositions. 2. To appreciate the significance of the psychological reduction in new light. To recognize the complementary role of the psychological propositions in explaining social phenomena by sociological propositions. In other words, to recoghize the necessity and significance of adding the explanation by psychological proposition to the explanation by sociological proposition. To give further explanation to sociological propositions in order to accomplish the sociological, efficient-cause, explanation of the social phenomena. I will try to make clear its necessity and its significance by citing an example of the origin of the institution of unilateral cross-coussin marriage. 3. Three difficulties of his theoretical system. a) The impression that his explanation is ad hoc. It is due to the arbitrariness in determining the givens which is used in the deductive system. b) The impression that his explanation make only a commonplace remark. It is due to the jump of steps in his reasoning from explanante to explicandum. Two difficulties given above seem to be gradualy eliminated as many sociologists share Homans' explanatory principles. c) The impression that his explanation is speculative. Although, when the value-proposition, or cost-proposition, is used as explanante, the fact that some reinforcers are felt as valuable, or costly, by the actor should be objectively proved, his explanation lacks this operation. The impression of speculativeness is due to the lack of this operation.