This article argues that what we now call public diplomacy emerged in the mid- to late 1930s in the case of Japan. It questions the notion that public diplomacy is new in contrast to 'traditional' diplomacy. It also questions the conventional understanding of Japan's diplomatic isolationism of the 1930s. The article argues that as a result of greater mass political participation, the idea of 'international public opinion' emerged as a new norm in inter-war international politics. States increasingly regarded news and cultural activities as crucial resources of their soft power for winning this international public opinion. Responding to technological developments in communications, they developed a more systematic approach to propaganda in order to utilize these resources in mainstream foreign policy. Even in the age of the socalled rise of nationalism and diplomatic isolationism, Japan could neither afford not to respond to other states' actions nor to ignore international public opinion. In the diplomatic crises of the 1930s, Japan began to coordinate news and cultural propaganda activities, and integrated them into a broader propaganda scheme. Here we see the origin of what is now called public diplomacy. This modern and internationalist thinking then prepared the institutional base for wartime propaganda.