Voluntary grassroots associations upgrade their structures to enable them to act organizationally and systematically as they grow in size. This fact was verified by studying volunteer groups in Japan. In particular, as the number of volunteers and funds involved increase, each group is motivated to establish a more formal and organized system. This present study also revealed that formalization continues to progress as the group focuses more closely on achieving its aims. Factors which stimulate systematization, other than size, will be further studied in the future.