What did Rousseau mean by "nature"? This is really one of such a kind of problem which is always both old and new. It is old because it seems to have been thoroughly disputed by many scholars of various branches of philosophical study. And yet, it remains new for us in the fact that Rousseau's "nature" cannot but be remembered and studied whenever the historical crises, political or educational, threaten us. Especially, among the pedagogues of Japan's Rousseau's "nature" has become something like an idol, and amid the educational chaos of present day Japan they are again hoisting it as a banner of their educational ideal. There are some dangers, however, in this passionate movement Some people identify it with a principle of "laissez-faire" and some understand it as a kind of barbarism and others keep it just like sentimental idol-worshippers do. All these erroneous understandings and movements result,from the ambiguity of the conception of "nature." They tried to explain it by making a classified table of its various meanings used in the Rousseau's works, or they thought it possible to throw light upon it by studying Rousseau's life or character more in details. But those strivings were all in vain. They could give us .nothing more than some perplexing informations about its ambiguity and complexity, and yet its true significance remains unknown. Isn't there any real understanding of "nature" which is one and may be the sole ground of our edticational activity? Did Rousseau inform us the one and true meaning of "nature" from the educational viewpoint? If not, why did he not or could he not describe it in a one and simple way so that he may not perplex his followers? We must find this reason, and if we can, we should lead to the ultimate understanding, of Rousseau's "nature," and accordingly to the true conception of the educational "nature." This report is a study to this purpose, that is to say, we tried to explain Rousseau's "nature," not by, as it were, a classifying metod but by faithfully following Rousseau's method of thinking.