This article analyses how diverse stances toward nuclear energy were produced and reproduced among Japanese labour unions from the mid-1960s to the mid-1980s, and how these stances of the formative period influenced the subsequent development of inter-union politics before and after the Fukushima nuclear plant disaster in 2011, based on content analysis of union bulletins and other materials of pro- and anti-nuclear energy unions. This article asks three research questions: (i) what factors contributed to pro- or anti-nuclear energy stances of labour unions?; (ii) what elements constituted or framed unions’ arguments on the issue of nuclear energy?; and (iii) what impact did these stances and arguments formed in the early period have on the subsequent development of inter-union politics over nuclear energy, especially in the aftermath of the Fukushima nuclear disaster? The paper shows union ideologies and institutional integration were the factors influencing unions’ stances on nuclear energy. It also analyses the worldviews of pro- and anti-nuclear energy unions based on the three policy styles of technological enthusiasts, moralists and cost-benefiters. The Fukushima disaster reinforced the previously formed division of union ideologies, but considerably weakened the discourse of technological enthusiasm stressed by pro-nuclear energy unions.