We performed μSR experiments on rubidium clusters arrayed in porous crystals of zeolite A. These clusters are known to show spontaneous magnetization of ferromagnetism at higher loading densities of Rb atoms in a previous study. The development of the internal field is observed below ≃ 4 K in the temperature dependence of zero-field μSR spectra in the sample n = 5.8, where n is the average loading density of Rb atoms per unit cage. The Curie temperature of this sample is estimated to be ≃ 5 K from the Arrott plot analysis. The amplitude of the relaxation is large enough to indicate the magnetic ordering in the major volume of sample. The internal field is confirmed to be almost static in the longitudinal-field μSR spectra below ≃ 4 K. Difference of the ferromagnetic properties between K and Rb clusters is also discussed from the viewpoint of μSR study.