The local effect of an interstitial hydrogenlike particle on localized f electrons was studied in PrPb3 by means of micro+ spin rotation and relaxation. Spontaneous micro+ spin precession with harmonic frequencies was observed for the first time in f electron compounds. We demonstrate that the signal is derived from a coupling between the micro+ spin and the hyperfine-enhanced nuclear spin of nearest neighbor (NN) 141Pr with Ising-like anisotropy. The signal also suggests a marked suppression of spin dynamics of the NN 141Pr in comparison with that of the bulk 141Pr. These facts strongly indicate modification of the f electronic state due to the interstitial charged particle.