J. S. Mill says “That an increase of the quantity of money raises prices, and a diminution lowers them, is the most elementary proposition in the theory of currency…” in his Principles of Political Economy (1848). However he should not be understood as a supporter of such “Simple Quantity Theory of Money.” He was rather an opponent of that theory. Because Mill was first a diciple and later a colleague of Thomas Tooke (1774-1858)—who is regarded as the strongest opponent of the “Simple Quantity Theory of Money.” Tooke's influence to Mill is shown clearly in his early papers, “Paper Currency and Commercial Distress” (1826) and “The Currency Question” (1844). In these papers, he demonstrates how the concept of “hoard” and the “law of reflux” invalidate the “Simple Quantity Theory.” Moreover he mentioned the two aspects of the circulation, the circulation of income and that of capital. These facts show that Mill held the same theoretical views as Tooke. And when his Principles is reread more carefully, it will be found that he did not revised this early position. My next subject is to make clear how he built those theoretical points into Principles and placed the role of money in it.