Using high-resolution spin-resolved photoemission spectroscopy, we observe a thermal spin depolarization to which all spin-polarized electrons contribute. Furthermore, we observe a distinct minority spin state near the Fermi level and a corresponding depolarization that seldom contributes to demagnetization. The origin of this depolarization has been identified as the many-body effect characteristic of half-metallic ferromagnets. Our investigation opens an experimental field of itinerant ferromagnetic physics focusing on phenomena with sub-meV energy scale.