The magnetism and structure for potassium clusters physisorbed in nanographite-based nanoporous host are investigated. Potassium clusters are subjected to a slight charge transfer to a host nanographite and modify the magnetism of the edge-state spins of nanographite. The localized spins of 4s electrons in the potassium cluster interact with each other through a strong antiferromagnetic interaction. Finite size effect and structural disorder around the cluster surface are responsible for anomalous spin fluctuations in the magnetism.