Yamagata Aritomo.
The Yamagata-Macmillan Treaty was a commercial and military treaty signed by Japan and Great Britain in 1901. The two island nations had grown closer in the wake of the Great Northern War of 1898-1901, which saw the United States of Mexico defeat the Russian Empire militarily and establish client states in Alaska and Siberia. The threat from the U.S.M. prompted the Meiji Emperor to end Japan's policy of isolation and seek increased trade and military assistance from the European powers. Britain proved to have the necessary funds and technology for the task, which led to increased trade between the two countries and culminated in the treaty.
Under the terms of the Yamagata-Macmillan Treaty, each nation promised to support the other in the event of war in the Pacific (presumably with the U.S.M.). The treaty was still in force twelve years later when French President Henri Fanchon was planning his war against the U.S.M. Sobel states that Japan and Great Britain remained allies at the time he was writing in 1971.
Sobel does not identify who the two signatories of the treaty were, but it is likely that Yamagata Aritomo signed on behalf of Japan, acting in his capacity as Foreign Minister.