A soft-magnetic material is indispensable for applying the magnetization dynamics of ferromagnetic metal to spintronic devices. One of the candidates is an amorphous ferromagnetic alloy. However, magnetization dynamics in the amorphous ferromagnetic alloy is disturbed in low magnetic field by various factors such as crystal fault, which is not preferable to the application. In this paper, we adopt a high-quality epitaxially grown ${rm Fe_3Si}$ as a soft-magnetic material and evaluate its magnetic property. As a result, we found that ${rm Fe_3Si}$ grown on Ge substrate shows good stable spin dynamics even in the low magnetic field.