Zero field (ZF) and transverse field (TF) muon spin relaxation and rotation (μSR) study has been carried out in filled-skutterudite SmOs4Sb12 in order to investigate the magnetically robust heavy-fermion (HF) state and the weak ferromagnetic anomaly appearing below ∼2.5K. A large-amplitude oscillating signal appears in the ZF-μSR spectra at low temperatures, confirming that the weak ferromagnetic anomaly is an intrinsic bulk property. Two components with the fraction ratio of ∼2:1 exist both in the ZF-μSR spectra of the ferromagnetically ordered state and the TF-μSR FFT spectra in applied fields along the 〈001〉 direction. This observation can be explained consistently based on the most probable muon stopping site indicated from the value of the static nuclear dipolar width in Kubo–Toyabe function in the high-temperature ZF-μSR spectra. Analysis reveals that the spontaneous magnetic moment Ms lies along the 〈001〉 direction and the size of Ms is largely suppressed, indicating that the weak ferromagnetic moment is carried by itinerant heavy quasiparticles.