  Non-adiabatic contribution of environmental degrees of freedom yields
effective inertia of spin in effective spin dynamics. In this paper, we study
several aspects of the inertia of spin in metallic ferromagnets. (i) a concrete
expression of the spin inertia $m_s$: $m_s=\hbar S_c/(2g_{\rm sd})$, where
$S_c$ is the spin polarization of conduction electrons and $g_{\rm sd}$ is the
$sd$ coupling constant. (ii) dynamical behavior of spin with inertia, discussed
from viewpoints of a spinning top and of a particle on a sphere. (iii) behavior
of spin waves and domain walls in the presence of inertia, and behavior of spin
with inertia in the case of a time-dependent magnetic field.
