Many amateur Japanese gardeners use small semi-public spaces to grow their favorite plants. These activities may be a kind of community development, but they do not always intend to contribute to such development. On the other hand, amateur gardeners have their own experiences and desires. What are the kinds of experiences contributing to building public space? Starting from this research question, the author conducts a case study on a semi-organized group at Shiba Municipal Park. From 2004, this group has been looking after and growing plants every Saturday morning as a volunteer activity, and then they keep a small flower garden to contribute to the local landscape of central Tokyo. The result of category relations shows the experiences as follows: the experiences of the members show a trend to deepen as they converge their way and knowledge of gardening. Then the experiences divide into two trends of deepening, one is the desire for the park to be appreciated and the other is a desire to look after and develop the park. The former experiences are easy to support and evaluate by local government, while the latter are important for the continuation of the group but difficult to support.