This article examines the two differing types of union strategy to organize part-time workers in Japan. One is a homogeneous strategy; the other, a heterogeneous strategy. Based on in-depth interviews with five enterprise labour unions, it was found that if both management and the union support part-time workers' up-skilling, active involvement and a sense of unity with regular workers for enhancing labour productivity, they tend to strategically choose the homogeneous strategy, which provides equal treatment to part-time workers in relation to human resource management practices and union participation. Under the homogeneous strategy, the company introduces a wage system that encourages part-time workers to work longer in order to obtain higher wages or to be promoted to a higher grade. Where management seeks to obtain a cost advantage from the use of part-time workers and the union puts priority on the protection of regular workers, they tend to choose the heterogeneous strategy, which discriminates against part-time workers in relation to both human resource management policy and union representation. The unionization of part-time workers, based on a homogeneous strategy, has positive effects, such as strengthening loyalty to the company and the union, part-time workers' up-skilling, and active involvement in workplace innovations. The unionization of non-regular workers can contribute to the revitalization of labour movements and the resolution of segmented labour market problems, as well as assisting in the recovery of corporate business and the national economy.