We report here the magnetic properties of the thulium-based reentrant superconductor Tm5Rh6Sn18 (T-c = 2.2 K), which were determined by magnetization and mu SR measurements. The spontaneous oscillation signal in the mu SR spectra gradually develops under zero magnetic field below 13 K. The development of a quasi-static local magnetic field is also observed even below T-c, indicating the coexistence of magnetism and superconductivity at zero magnetic field. From the analysis of longitudinal mu SR measurements, magnetic ordering can be considered to be not truly long range but superparamagnetic.