Around 1930, the Japanese publishing market was restructured, and as part of this process, the colonial market emerged within the Japanese Empire. In an attempt to expand into the colonial market, publishers such as Kaizō-sha, Chūōkōron-sha, and Senki-sha competed among each other, producing ‘legal’ and ‘illegal’ commodities related to socialism. This paper examines the circulation of illegal commodities such as the often-banned magazine Senki (Battle Flag), cross-reading them with internal documents from Senki-sha (Senki’s publisher) and NAPF (All-Japan Federation of Proletarian Arts), as well as with those from the Japanese Home Ministry and the Japanese Government-General in Korea. By doing so, the essay argues that the main actors of the socialist cultural movement around 1930 purposefully planned to capitalize on the ‘illegal’ nature of their commodities, while adopting a public stance of differentiation from commercial capital. Furthermore, by proposing that the publication of illegal commodities was in fact deeply imbricated with the movement of capital in the publishing market, this paper also reveals that Korean-language publications–notably, the magazine Uri tongmu (Our Comrades)–produced by socialists in the Japanese interior around 1930, ended up playing a role in undermining the reconstruction of socialism in Korea. For this reason, it is crucial to reconsider the prevailing narrative about the history of the Japanese socialist movement of the late 1920s and early 1930s, which often essentializes the connection between Japanese and Korean socialists as pure ideological solidarity, paying little attention to the complex movement of capital, legal and illegal, at work in the Japanese Empire around 1930.