Zeolites are porous crystals that make it possible to generate regularly arrayed alkali-metal nanoclusters. Potassium clusters arrayed in zeolite A show ferromagnetism below 7 K without any magnetic elements. This is a novel magnetic material because the magnetic order is realized by exchange interactions between s-electrons of alkali metals. Because of the small ferromagnetic moment (0.3 muB per cluster) and an antiferromagnetic interaction observed in magnetization measurements, the system is not regarded as a simple ferromagnet. Although a spin-canted antiferromagnetic model and a ferrimagnetic model have been proposed, it has not been clarified experimentally yet. It is scientifically significant to clarify the reason why the s-electron system can show ferromagnetism. In this work we approach this issue by directly solving the magnetic structure using neutron diffraction.