This study aims to examine the roles of a support organization in a privately developed rental collective housing project, "Kankanmori", focusing on the concept of "community of practice (COP)". Engaging in joint activities and discussions, the resident group developed a set of shared knowledge and value as practices for "collective living with collaboration and pleasure". The support organization played a role in cultivating the COP by giving support according to the autonomy of the COP and the timeline of the design and construction.