In the recent studies on urban public space, despite their analytical importance, the roles of strangers in the process of improvising the publicness of space have not been well examined. This paper explores how people engaged themselves in the political discussion with strangers in Shinjuku West Exit Underground Plaza in the late 1960s. Previous research have argued that the antiwar activists, folk guerrillas, and the urban structure of Tokyo stimulated the passersby to debate. These approaches, however, cannot explain how the debate had begun a year before the guerrillas appeared. The question is how the passersby in the plaza could form and continue the discussion sessions with strangers. This article perceives the street discussion in the plaza as the spontaneous interaction among strangers and explicates its process through highlighting their spatial techniques of debate in urban space. At first, some student radicals were collecting donations and signatures in the plaza and some passersby asked them why they adopted violent ways of protest. Then another commuter supported and defended the students, which eventually lead to the debate among passersby. This process was catalyzed by the makeshift stalls set up around the pillars across the plaza by some fundraisers and vendors. When the discussion reached a deadlock, some onlookers and hecklers surrounding the debaters intervened in and moderated the heated conversation. People utilized the architecture of the plaza to watch the sessions and joined the discussion in their own ways. In conclusion, the dynamic formation of discussants, listeners, and moderators through the appropriation of urban space and the conversation among passersby will be elucidated. By emphasizing the interaction among strangers and their spatial techniques of discussion, this article critically develops the studies on the publicness of urban space, which have centered on the organized activism or the everyday interaction.